Skip to main content
Select Source:

Endocrine System

Endocrine system

The endocrine system is the human body's network of glands that produce more than 100 hormones to maintain and regulate basic bodily functions. Hormones are chemical substances carried in the bloodstream to tissues and organs, stimulating them to perform some action. The glands of the endocrine system include the pituitary, pineal, thyroid, parathyroids, thymus, pancreas, adrenals, and ovaries or testes.

The endocrine system oversees many critical life processes. These involve growth, reproduction, immunity (the body's ability to resist disease), and homeostasis (the body's ability to maintain a balance of internal functions). The branch of medicine that studies endocrine glands and the hormones they secrete is called endocrinology.

Hormonal levels in the blood

Most endocrine hormones are maintained at certain levels in the plasma, the colorless, liquid portion of the blood in which blood cells and other substances are suspended. Receptor cells at set locations throughout the body monitor hormonal levels. If the level is too high or too low, the gland responsible for its production is notified and acts to correct the situation. Most hormones have this type of regulatory control. However, a few hormones operate on a system whereby high levels of the particular hormone activate the release of another hormone. The end result is usually that the second hormone will eventually decrease the production of the initial hormone.

The pituitary

The pituitary gland has long been called the master gland because it regulates many other endocrine glands. It secretes multiple hormones that, in turn, trigger the release of other hormones from other endocrine sites. The pituitary is located at the base of the brain behind the nose and is separated into two distinct lobes, the anterior pituitary (AP) and the posterior pituitary (PP). The entire pituitary hangs by a thin piece of tissue, called the pituitary stalk, beneath the hypothalamus (the region of the brain controlling temperature, hunger, and thirst).

The pituitary secretes at least five hormones that directly control the activities of other endocrine glands. These are thyrotropic hormone (affecting the thyroid gland), adrenocorticotropic hormone (affecting the adrenal cortex), and three gonadotropic hormones (affecting the reproductive glands).

Words to Know

Carbohydrate: A compound consisting of carbon, hydrogen, and oxygen found in plants and used as a food by humans and other animals.

Hormones: Chemical substances secreted by endocrine glands that are carried in the bloodstream to tissues and organs, stimulating them to maintain and regulate basic bodily functions.

Metabolism: Sum of all the physiological processes by which an organism maintains life.

Plasma: Colorless, liquid portion of the blood in which blood cells and other substances are suspended.

The pituitary also secretes hormones that do not affect other glands, but control some bodily function. These include somatotropic or growth hormone (which controls growth in all tissues) and antidiuretic hormone (which controls the amount of water excreted by the kidneys).

The pineal

The pineal gland or body is a small cone-shaped gland believed to function as a body clock. The pineal is located deep in the rear portion of the brain. It secretes the hormone melatonin, which fluctuates on a daily basis with levels highest at night. Scientists are not quite sure of the role of melatonin. Some believe it plays a role in the development of the male and female sex glands.

The thyroid

The thyroid is a butterfly-shaped gland that wraps around the front and sides of the trachea (windpipe). The thyroid is divided into two lobes connected by a band of tissue called the isthmus. Thyroid hormones play several important roles in growth, development, and metabolism. (Metabolism is the sum of all the physiological processes by which an organism maintains life.) The major hormones produced by the thyroid are thyroxine and calcitonin. Thyroxine controls the metabolic rate of most cells in the body, while calcitonin maintains proper calcium levels in the body.

The parathyroids

The parathyroids are four small glands (each about the size of a pea) located behind the thyroid gland. These glands secrete parathormone, which regulates calcium (and phosphate) levels in the body. Calcium has numerous important bodily functions. It makes up 2 to 3 percent of the weight of the average adult. Roughly 99 percent of the calcium in the body is contained in the bones. Calcium also plays a pivotal role in muscle contraction.

The thymus

The thymus is located in the upper part of the chest underneath the breastbone. In infants, the thymus is quite large. It continues to grow until puberty, when it begins to shrink. The size of the thymus in most adults is very small. Like some other endocrine glands, the thymus has two lobes connected by a stalk. The thymus secretes several hormones that promote the development of the body's immune system.

The pancreas

The pancreas is a large gland situated below and behind the stomach in the lower abdomen. The pancreas secretes pancreatic juice into the duodenum (the first section of the small intestine) through the pancreatic duct. The digestive enzymes in this juice help break down carbohydrates, fats, and proteins.

Scattered among the cells that produce pancreatic juice are small groups of endocrine cells. These are called the Islets of Langerhans. They secrete two hormones, insulin and glucagon, which maintain blood glucose (sugar) levels.

Insulin is secreted in response to high glucose levels in the blood. It lowers sugar levels in the blood by increasing the uptake of glucose into the tissues. Glucagon has the opposite effect. It causes the liver to transform the glycogen (a carbohydrate) it stores into glucose, which is then released into the blood.

The adrenals

The adrenals are two glands, each sitting like a cap on top of a kidney. The adrenals are divided into two distinct regions: the cortex (outer layer) and the medulla (inner layer). The cortex makes up about 80 percent of each adrenal. The adrenals help the body adapt to stressful situations.

The cortex secretes about 30 steroid hormones. The most important of these are cortisol and aldosterone. Cortisol regulates the body's metabolism

of carbohydrates, proteins, and fats. Aldosterone regulates the body's water and salt balance. The cortex is extremely important to bodily processes. If it stops functioning, death occurs in just a few days.

The medulla secretes the hormones adrenaline and noradrenaline. Both of these hormones are released during dangerous or stressful situations. They increase heart rate, blood pressure, blood flow to the muscles, blood sugar levels, and other processes that prepare a body for vigorous action, such as in an emergency.

The ovaries

In females, the ovaries are located at the end of each fallopian tube and are attached to the uterus by an ovarian ligament. They produce the female reproductive hormones estrogen and progesterone. These hormones work together with the gonadotropic hormones from the pituitary to ensure fertility. They are also important for the development of sexual characteristics during puberty.

Each month after puberty, increased levels of estrogen signal the pituitary gland to secrete luteinizing hormone (LH; a gonadotropic hormone). Once LH is secreted, the ovaries release a single egg (a process called ovulation). While an egg travels down the fallopian tube, progesterone is released, which prevents another egg from beginning to mature. The egg then attaches to the lining of the uterus. If fertilization does not occur, the egg (with the lining of the uterus) is shed outside the body during the monthly process called menstruation.

During pregnancy, high levels of estrogen and progesterone prevent another egg from maturing. In addition, progesterone prevents the uterus from contracting so that the developing embryo is not disturbed, and helps to prepare breasts for lactation (the formation and secretion of milk).

At menopause, which usually occurs between the ages of 40 and 50, estrogen levels fall dramatically and the monthly cycle of ovulation and menstruation comes to an end.

The testes

The two testes are located in the scrotum, which hangs between the legs behind the penis. In addition to producing sperm, the testes produce testosterone, the principal male sex hormone. At puberty, increased levels of testosterone bring about the development of sexual characteristics (increased genital growth, facial hair, voice change). Testosterone helps sperm to mature and aids in muscular development. After about the age of 40, testosterone levels gradually decline.

Endocrine disorders

As much as 10 percent of the population will experience some endocrine disorder in their lifetime. Most endocrine disorders are caused by an increased or decreased level of particular hormones. Tumors (abnormal tissue growth) in endocrine glands are one of the major causes of hormone overproduction. Hormone underproduction is often due to defective receptor cells, which fail to notify an endocrine gland when productive of its particular hormone is too low. Injury or disease can also result in low hormone levels.

The overproduction of the growth hormone can cause giantism (unusually large stature). Underproduction of the same hormone can lead to the opposite condition, dwarfism. A similar disorder, cretinism, occurs when the thyroid does not produce enough calcitonin, which is necessary for bone growth. Addison's disease is a rare condition caused by insufficient hormone production by the adrenal cortex. It is characterized by extreme weakness, low blood pressure, and darkening of the skin and mucous membranes. Low insulin production by the Islets of Langerhans can result in diabetes mellitus, a condition marked by excessive thirst, urination, and fatigue. If left untreated, diabetes can cause death.

[See also Diabetes mellitus; Hormone ]

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Endocrine System." UXL Encyclopedia of Science. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"Endocrine System." UXL Encyclopedia of Science. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/endocrine-system-1

"Endocrine System." UXL Encyclopedia of Science. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/endocrine-system-1

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Endocrine System

Endocrine System

The endocrine system is part of the regulatory system in animals and helps maintain the internal balance of the body. Both vertebrates and invertebrates have endocrine systems. The endocrine system regulates many functions of the body, including growth and metabolism, water balance, sugar and calcium balance in the bloodstream, and several functions related to sexual maturity and reproduction. Two major functions under endocrine control in invertebrates are the shedding of the exoskeleton for growth, called molting, and metamorphosis, functions that do not occur in vertebrates.

The endocrine system is not as fast to respond to stimuli as is the nervous system (the other major regulatory system in animals), which can respond in less than a second. The endocrine system can respond within minutes, and the effects usually last longer than the effects of the nervous system.

The endocrine system is made up of organs that produce chemical messengers called hormones. Hormones are released directly into the bloodstream in vertebrates and the haemolymph in invertebrates. Hormones circulate with the blood, so they are everywhere in the body.

Only certain cells, however, are capable of responding to these chemical messengers. These are target cells, which have special receptors for different kinds of hormones. Every chemical messenger has a unique shape. The target cell has a receptor that corresponds to the shape of the messenger. Most receptors are outside of a cell, embedded in the cell membrane.

When a messenger binds to the target, a different messenger is released inside of the cell. This second signal inside the cell is called a secondary messenger. This secondary messenger then triggers other changes inside the cell, such as the release of a substance. Other target cells have receptors on the inside of the cells. Specifically, some hormones can go inside of the cell and bind to a receptor that turns on and off DNA transcription of specific genes.

Endocrine Organs and Effects

The endocrine system works through the same process in vertebrates and invertebrates, although the organs and chemical messengers involved differ. In invertebrates, the nervous system has modified cells that secrete most types of hormones. The hormones released from within the nervous system regulate the other endocrine organs in invertebrates.

These other organs include the corpora cardiaca, the prothoracic glands, and the corpora allata. The corpora cardiaca are located next to the brain and secrete hormones that control the prothoracic glands. The prothoracic glands are located behind the brain and secrete ecdysone, which stimulates and controls molting, as well as other hormones involved in the molting process. The corpora allata is located near the digestive system and secretes juvenile hormone. Juvenile hormone is involved in growth, metamorphosis, and reproduction. The gonads (ovaries and testes) also secrete hormones in the invertebrates and are involved in reproduction.

Vertebrates have more endocrine organs than invertebrates. The hypothalamus, pituitary gland, and pineal gland are located in the brain. The hypothalamus controls the pituitary gland. The pituitary gland controls water regulation and endocrine production of the gonads, and stimulates growth as well. The pineal gland controls biological rhythms such as sleep by producing melatonin.

All other endocrine organs are located in the body cavity. The pancreas controls blood sugar levels by secreting two hormones that have the opposing functions of raising and lowering blood sugar levels. The thyroid and parathyroid control calcium levels in a manner similar to the pancreas: one hormone raises calcium and another lowers calcium levels. The thyroid also controls metabolism. The adrenal glands are located above the kidney. They are involved in both long-term and short-term stress responses. The thymus is involved in immune responses. The gonads are involved in many functions.

The gonads consist of the ovaries and testes and in vertebrates control development and growth in addition to regulating reproduction. The gonads secrete steroid hormones. Steroids are one of the chemical messengers that have receptors inside of target cells and most cells have steroid receptors, so that steroids affect the entire body.

The gonads produce three classes of steroid hormones: androgens that include testosterone, estrogens, and progestins. Both testes and ovaries produce all three steroid types, but in different proportions. In humans, steroids determine the sex of a fetus during development. If androgens are present at high levels during fetal development, then the fetus develops as a male. If androgens are not present at high levels, then the fetus develops as a female.

Steroids are also responsible for sexual maturation and the development of secondary sex characteristics during puberty in humans. Secondary sex characteristics in males caused by high levels of androgens include changing patterns in hair growth such as baldness and facial hair growth and deepening of the voice. Estrogen in females cause secondary sex characteristics such as the development of breasts. Progestins in females cause reproductive cycles and menstruation.

Supplemental Hormones for Humans

Humans sometimes take hormones by pills or injections to alter or supplement the body's own production of hormones. The best example of necessary hormone supplements is insulin replacement for diabetes mellitus. The pancreas secretes insulin, which lowers blood sugar levels, and glucagon, which raises blood sugar levels. When someone is diabetic, the body does not produce enough insulin and blood sugar remains at too high a level for normal water and metabolic functions.

Type I diabetes mellitus starts during childhood and is an autoimmune disease. Someone with Type I diabetes mellitus must take injections of insulin to control blood sugar levels. The insulin is either extracted from the organs of other animals or is produced by bioengineering bacteria to produce insulin. Those having Type II diabetes mellitus are often over forty years old and can control blood sugar levels with special diets and exercise.

Another common form of hormones taken by humans is steroids. A practice that is neither legal nor safe is that of individuals, usually males, taking steroids to increase muscle growth. Athletes of all types do this, not just bodybuilders. Androgens facilitate the acquisition of muscle mass, which is why men are more muscular than are women. However, taking supplemental androgens will cause the body to shut down its own production of androgens and interfere with the body's reproductive functions.

Common side effects of taking androgens include shrinking of the testes, impotence, the development of female secondary sex characters such as breasts, and a serious risk of heart attack. Additionally, sources for these androgens are usually other animals such as horses and illegal androgens are often impure, containing antibodies from the source animals. These antibodies can cause severe immune responses in humans and can even be fatal.

Birth control pills are another common form of steroids taken by humans. Birth control pills contain man-made estrogens and progestins. Birth control pills prevent ovulation, the development and release of an egg by the female, by disrupting the normal cycle of hormones that comprise the female menstrual cycle.

Environmental estrogens are chemicals that are thought to function as chemical messengers in animals. Examples of environmental estrogens include plastics and by-products of manufacturing. It is not completely understood at this point whether or not environmental estrogens can affect animals, and if so, to what degree.

Laura A. Higgins

Bibliography

Crews, David. "Animal Sexuality." Scientific American 270 (January 1994):108-115.

Johnson, George B. Biology: Visualizing Life. New York: Holt, Rinehart and Winston Inc., 1998.

McLachlan, John A., and Steven F. Arnold. "Environmental Estrogens." AmericanScientist 84 (September-October 1996):452-461.

Steinman, L. "Autoimmune Disease." Scientific American 269 (September 1993): 106-114.

Tjian, R. "Molecular Machines That Control Genes." Scientific American 272 (February 1995):54-61.

Haemolymph is the "blood" of invertebrates. Similar to mammalian blood, it carries nutrients to cells and waste away from cells. Haemolymph does not carry oxygen to cells like mammalian blood.

Hemolymph functions the same way as haemolymph, but works for insects instead of invertebrates.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Endocrine System." Animal Sciences. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"Endocrine System." Animal Sciences. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/science/news-wires-white-papers-and-books/endocrine-system

"Endocrine System." Animal Sciences. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/news-wires-white-papers-and-books/endocrine-system

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Endocrine System

Endocrine System

The endocrine system is the interacting group of glands that secrete hormones , helping to control cells and organs throughout the body. How do cells and organs at different locations in the body communicate with each other to maintain the physiology of healthy living organisms? What happens if organs do not communicate properly? These questions can be answered by understanding how organs of the nervous system and endocrine system function.

There are similarities and differences between how the human nervous system and endocrine system communicate with and control other organs. For example, the nervous system relies on electrical impulses and chemical neurotransmitters . Most endocrine organs do not transmit electrical information but instead secrete hormones (from the Greek, meaning "to arouse or excite"), which are molecules that act as chemical messengers. Hormones are released into the bloodstream whereby they travel to organs they affect, known as target organs.

Endocrine organs are located throughout the body, and they have diverse functions controlling events such as cell metabolism , blood sugar concentration, digestion, the menstrual cycle in females, and the production of male and female gametes . Primary endocrine organs include the hypothalamus, pituitary gland, pineal gland, thyroid and parathyroid glands, thymus, adrenal glands, pancreas, and male and female gonads, the testes and ovaries respectively. Other tissues serve endocrine functions through the hormones they produce. For example, the kidneys produce erythropoietin that stimulates formation of red blood cells, and the skin produces vitamin D, a steroid derivative required for calcium absorption by the small intestine.

Hormones

Hormones are "signaling" molecules because they influence the activity of other cells that may be far from where the hormone was produced. For a hormone to affect a target cell, it must attach to a receptor protein on the target cell membrane or inside the cell. Hormone binding to a receptor triggers an intricate set of biochemical interactions that can affect the target cell in myriad ways. For example, hormones can influence cell metabolism, cell division, electrical activity, ribonucleic acid (RNA) and protein synthesis, or cell secretion .

There are several different types of hormones that vary in their chemical organization and functions. The majority of hormones are peptides. These consist of short sequences of amino acids ; examples include insulin and growth hormone. The class of hormones called steroids are synthesized from cholesterolexamples include male sex steroids such as testosterone and female sex steroids such as estrogen and progesterone.

Hormone production by an endocrine organ is regulated by complex interactions, called feedback loops, between the endocrine organ and its target organs. Feedback loops are two-way modes of communication in which a target organ also releases molecules that regulate the endocrine organ. Feedback loops are designed to maintain hormone concentration within a normal range. Endocrine disorders in which hormone concentration becomes abnormal can be difficult to diagnose and treat because of the complexity of feedback loops. One simple way to classify endocrine disorders is based on whether a condition is due to excess production (hypersecretion) or underproduction (hyposecretion) of hormone.

The Major Endocrine Glands

Located at the base of the brain, the pituitary gland produces many hormones that regulate other organs. Because of this, the pituitary is often referred to as the "master" endocrine gland, although the term "central" endocrine gland is more correct because hormone release by the pituitary is primarily regulated by a brain structure called the hypothalamus, which acts to connect the nervous system to the endocrine system. The hypothalamus produces hormones that stimulate or inhibit the release of pituitary hormones. The hypothalamus also produces antidiuretic hormone, which regulates water balance in the body by inhibiting urine formation by the kidneys, and a hormone called oxytocin, which stimulates uterine contractions during childbirth and releases milk during breast-feeding.

Hormones released by the pituitary include growth hormone, which increases during childhood and stimulates the growth of muscle, bone, and other tissues. Sporadic bursts in growth hormone release often result in rapid growth "spurts" associated with adolescence. Hyposecretion of growth hormone can result in dwarfism, whereas hypersecretion of growth hormone can cause gigantism and other disorders. The pituitary also produces follicle-stimulating hormone and luteinizing hormone, which stimulate gamete production and sex steroid production in male and female reproductive organs, and prolactin, which stimulates milk formation in the mammary glands.

Located adjacent to the larynx , the thyroid gland primarily produces thyroxine and triiodothyronine, collectively referred to as thyroid hormone. Thyroid hormone stimulates growth of muscles and bones, carbohydrate metabolism, and basal metabolic rate. Its production requires iodine; the lack of dietary iodine causes goiter, a thyroid gland that is overly enlarged in an effort to compensate for the thyroid hormone deficiency.

Effects of thyroid disorders in children and adults can differ widely. For example, hyposecretion of thyroid hormone in infants causes congenital hypothyroidism, a disease characterized by mental retardation and poor body growth; hyposecretion in adults produces myxedema, with symptoms such as lethargy , weight gain, and dry skin. Conversely, hypersecretion of thyroid hormone in adults causes Graves' disease, a condition characterized by weight loss, nervousness, and dramatic increases in body metabolism. The thyroid also produces calcitonin, a hormone that regulates blood calcium concentration.

The adrenal glands are small organs on the apex of each kidney. The outer layers of cells in the adrenal gland, called the adrenal cortex, produce several hormones that affect reproductive development; mineral balance; fat, protein, and carbohydrate balance; and adaptation to stress. The inner part, called the adrenal medulla, secretes epinephrine and norepinephrine, which activate the sympathetic nervous system and stimulate the "fight-or-flight" response that helps the body cope with stressful situations, such as fear.

The pancreas produces insulin and glucagon, which function in opposing fashion to regulate blood sugar (glucose) concentration. When blood glucose level risesfor example, after eating a sugar-rich mealinsulin lowers it by stimulating glucose storage in liver and muscle cells as long chains of glucose called glycogen . Conversely, between meals, blood glucose level decreases. In response, the pancreas releases glucagon, which stimulates glycogen breakdown and subsequent release of glucose into the bloodstream. One of the most well characterized endocrine disorders is diabetes mellitus, resulting from hyposecretion of insulin or, more commonly, target cell insensitivity to it.

Endocrine functions of the gonads are addressed in articles on the male and female reproductive systems. The sex hormone testosterone regulates sperm production in males. Estrogen and progesterone influence egg maturation and release (ovulation) and control the uterine (menstrual) cycle in females.

Although the many hormones produced by human endocrine organs have a wide variety of actions, the common purpose of all hormones is to facilitate organ-to-organ communication necessary for body physiology.

see also Adrenal Gland; Anabolic Steroids; Blood Sugar Regulation; Female Reproductive System; Growth; Homeostasis; Hormones; Hypothalamus; Nervous Systems; Pancreas; Pituitary Gland; Stress Response; Thyroid Gland

Michael A. Palladino

Bibliography

Hadley, Mac E. Endocrinology, 5th ed. Upper Saddle River, NJ: Prentice Hall, 2000.

Marieb, Elaine Nicpon. Human Anatomy and Physiology, 5th ed. San Francisco: Benjamin Cummings, 2000.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Endocrine System." Biology. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"Endocrine System." Biology. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/science/news-wires-white-papers-and-books/endocrine-system-0

"Endocrine System." Biology. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/news-wires-white-papers-and-books/endocrine-system-0

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Endocrine System

ENDOCRINE SYSTEM

Among the most intellectually compelling theories of aging are those based on the notion that selection pressure favors those mechanisms which increase the probability of reproductive success (i.e., of producing a next generation of viable offspring). This relationship would be true even if, for example, the mechanisms utilized early in life to assure reproductive success ultimately contribute to senescence and reduce the longevity of postreproductive-age adults (Rose). The dual and apparently contradictory nature of this particular proposal is reflected in the term used to describe it, "antagonistic pleiotropy." Similar to antagonistic pleiotropy is another hypothesis called the "disposable soma." According to this view, the soma (body) is principally a vehicle for reproduction that becomes disposable once reproductive success has been achieved. In practice, this means that the resources of the body are targeted to assuring the perpetuation of the species instead of being distributed in a manner that may also help increase life span (Kirkwood).

Reproduction, including the antecedent development of reproductive organs and secondary sexual characteristics, is under strong hormonal control. If aging and life span are linked to the cessation of reproductive activity, as proposed above, then it follows that hormones likely play an important role in these events. In humans the most prominent sex hormones are the steroids estrogen (estradiol), progesterone, and testosterone. Of these, estrogen has probably received the greatest attention as a putative regulator of the aging process. Estrogen availability and levels determine the occurrence of two landmark events in the life cycle of women, menarche and menopause, and its virtual disappearance in postmenopausal women is associated with a variety of disorders and regressive tissue changes commonly associated with aging (Perry). These associations include the loss of bone (osteoporosis) and changes in skin texture, and the increased risk of cataracts, cardiovascular disease, and dementia. In addition, because of the reciprocal relationship between hormone levels and, for example, bone loss, estrogen has been used extensively in hormone replacement therapy (HRT), the goal being to counteract the negative effects of postmenopausal estrogen deficiency, that is, those changes commonly associated with aging (Palacios). Thus, by this criterion, estrogen is an antiaging hormone.

While estrogen is almost certainly the most widely recognized sex hormone that likely plays a role in aging, it is not the only sex hormone that has this distinction. There are two others that also have gained significant attention. One of these is testosterone, which was recognized many years ago, albeit indirectly, as an agent (factor) important in maintaining the vigor and general vitality of older males. This association had its origins in nineteenth-century research involving testicular transplants in animals (the goal being to restore the sexual activity of valuable old, stud animals), and progressed to the use of both transplants and testicular extracts in middle-aged to elderly human males (Gosden). This early work gave results that were at best equivocal, but it did establish the basis for research on hormone isolation and characterization, and the rationale for the use of testosterone in HRT. Testosterone levels in men do decline with age (the phenomenon is called andropause ), and low levels of the hormone have been equated with the loss of libido and cognitive skills, physical frailty, and bone loss. Testosterone HRT is an accepted form of therapy, particularly for men who are clearly hypogonadal (defined as bioavailable testosterone below the reference range for young men), but its widespread use remains controversial because of side effects (Bain).

The other sex steroid that has been widely implicated in the aging process is dehydroepiandrosterone (DHEA). DHEA is a weak androgen, produced by the adrenal gland, that can act directly on target tissues or indirectly as a precursor molecule for both estrogen and testosterone. DHEA, and its sulfated derivative DHEA-S, reach peak levels in young adults and decline steadily thereafter in most individuals. The blood levels at age eighty-five are, on average, about 10 percent of that in young adults. This diminution in DHEA/DHEA-S is called adrenopause. Lower levels of the hormone have been associated with age-related changes in body composition, the frequency of some forms of cancer, type II diabetes, atherosclerosis, and ischemic heart disease (Hinson and Raven). As is the case with estrogen and testosterone, DHEA has been and is being used in HRT, frequently in uncontrolled circumstances by the public at large in the United States. Although data from animal studies indicate that DHEA supplementation can counter some age-related changes (e.g., in immune function), the results on healthy, older humans leave the question of benefit in doubt (Svec and Porter). However, DHEA may be of help in some medical conditions, including serum lupus erythematosis and serious depression. No adverse effects have been reported.

No discussion of hormones and aging would be complete without consideration of human growth hormone (HGH) and insulin-like growth factor-1 (IGF-1). HGH is produced by the pituitary and affects target tissues principally through a mediating hormone/cytokine, IGF-1, that is synthesized in peripheral tissue. HGH and IGF-1 are potent anabolic agents capable of stimulating cell proliferation and protein synthesis, and recombinant HGH is the intervention of choice for treating individuals with short stature and adult HGH deficiency. HGH and IGF-1 levels decline with age (somatopause ) (Vermeulen), and HGH and HGH secretagogue therapy have emerged as strategies for helping the frail elderly regain strength and muscle mass. However, as is the case with testosterone, it remains to be seen whether the benefits of such an approach outweigh the risks to the individuals, including peripheral edema and a decrease in insulin sensitivity (Cummings and Merriam). Ironically, recent data show that dwarf mice with growth hormone deficiency and, as a consequence, reduced body size live longer than their normal littermates.

Arnold Kahn

See also Androgen; Andropause; Biomarkers; DHEA; Estrogen; Growth Hormone; Insulin; Longevity, Reproduction; Menopause; Neuroendocrine System; Theories of Biological Aging: Disposable Soma.

BIBLIOGRAPHY

Bain, J. "Andropause. Testosterone Replacement Therapy for Aging Men." Canadian Family Physician 47 (2001): 9197.

Cummings, D. E., and Merriam, G. R. "Age-Related Changes in Growth Hormone: Should the Somatopause Be Treated?" Seminar in Reproductive Endocrinology 17, no. 4 (1999): 311325.

Gosden, R. Cheating Time: Science, Sex and Aging. New York: W. H. Freeman, 1996.

Hinson, J. P., and Raven, P. W. "DHEA Deficiency Syndrome: A New Term for Old Age?" Journal of Endocrinology 163 (1999): 15.

Kirkwood, B. L. "Evolution of Aging." Nature 270 (1977): 301304.

Palacios, S. "Current Perspectives on the Benefits of HRT in Menopausal Women." Maturitas 33, supp. 1 (November 1999): S1S13.

Perry, H. M., III "The Endocrinology of Aging." Clinical Chemistry 45, no. 8 (pt 2) (1999): 13691376.

Rose, M. R. Evolutionary Biology of Aging. New York: Oxford University Press, 1991.

Svec, F., and Porter, J. R. "The Actions of Exogenous Dehydroepiandrosterone in Experimental Animals and Humans." Proceedings of the Society for Experimental Biology and Medicine 218 (1997): 174191.

Vermeulen, A. "Andropause." Maturitas 34, no. 1 (2000): 515.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Endocrine System." Encyclopedia of Aging. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"Endocrine System." Encyclopedia of Aging. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/education/encyclopedias-almanacs-transcripts-and-maps/endocrine-system

"Endocrine System." Encyclopedia of Aging. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/education/encyclopedias-almanacs-transcripts-and-maps/endocrine-system

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

endocrine system

endocrine system (ĕn´dəkrĬn), body control system composed of a group of glands that maintain a stable internal environment by producing chemical regulatory substances called hormones. The endocrine system includes the pituitary gland, thyroid gland, parathyroid glands, adrenal gland, pancreas, ovaries, and testes (see testis). The thymus gland, pineal gland, and kidney (see urinary system) are also sometimes considered endocrine organs.

The endocrine glands appear unique in that the hormones they produce do not pass through tubes or ducts. The hormones are secreted directly into the internal environment, where they are transmitted via the bloodstream or by diffusion and act at distant points in the body. In contrast, other glands including sweat glands, salivary glands, and glands of the gastrointestinal system secrete the substances they produce through ducts, and those substances are used in the vicinity of the gland.

The regulation of body functions by the endocrine system depends on the existence of specific receptor cells in target organs that respond in specialized ways to the minute quantities of the hormonal messengers. Some endocrine hormones, such as thyroxine from the thyroid gland, affect nearly all body cells; others, such as progesterone from the female ovary, which regulates the uterine lining, affect only a single organ. The amounts of hormones are maintained by feedback mechanisms that depend on interactions between the endocrine glands, the blood levels of the various hormones, and activities of the target organ. Hormones act by regulating cell metabolism. By accelerating, slowing, or maintaining enzyme activity in receptor cells, hormones control growth and development, metabolic rate, sexual rhythms, and reproduction.

Pituitary Control

The master gland, i.e., the gland that regulates many of the other endocrine glands, is the pituitary, located at the base of the brain. Also called the hypophysis, the pituitary secretes at least five hormones that directly affect the other endocrine glands. It secretes thyrotropin, which manages thyroid gland activity, adrenocorticotropic hormone (ACTH), which regulates activity of the adrenal cortex, and three gonadotropic hormones, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and luteotropic hormone (LTH), all of which control the growth and reproductive activities of the sex glands. The pituitary also produces substances that do not act directly on other endocrine glands: somatotropic hormone, or growth hormone, which controls growth in all tissues; antidiuretic hormone (ADH), which controls the rate of water excretion in the urine; oxytocin, which stimulates uterine contraction and helps regulate milk production by the breasts; and melanocyte-stimulating hormone, which regulates the activity of the melanocytes, or pigment-producing cells.

Adrenal Gland

The adrenal gland is another endocrine gland regulated by the pituitary. The adrenal cortex, the outer part of each of the two adrenal glands, produces aldosterone, cortisol, and other steroids. These substances regulate salt concentration in body fluids and glucose, fat, and protein metabolism. The inner portion of the gland, the adrenal medulla, secretes epinephrine (adrenaline) and norepinephrine, substances connected with the autonomic nervous system that help the body to respond to danger or stress.

The Thyroid Gland

The thyroid, located below the larynx and partially surrounding the trachea, produces thyroxine, which controls the metabolic rate of most body cells, and calcitonin, which is responsible for maintaining proper calcium serum levels in the body.

The Sex Hormones

The testes produce the male sex hormone testosterone, which controls the development of the male sex organs as well as secondary sex characteristics. The pituitary hormone LH regulates testosterone production, and FSH initiates sperm formation in the testes. In females, FSH, LH, and LTH are integrated into the complex monthly cycles of ovulation, production of the hormones estrogen and progesterone by the ovaries and corpus luteum, and menstruation; LTH also contributes to lactation. Estrogen controls growth of the sex organs and breasts and regulates secondary sex characteristics. The most important function of progesterone is to prepare the uterine lining for implantation of a fertilized egg.

Other Endocrine Glands

The other endocrine glands are not directly controlled by the pituitary. The four parathyroid glands, located behind the thyroid, secrete a hormone that regulates calcium and phosphate metabolism. The endocrine portion of the pancreas, called the islets of Langerhans, secretes insulin, which regulates the level of sugar (glucose) in the blood and glucagon, which raises blood sugar level. The thymus, sometimes considered another endocrine gland, processes lymphocytes in newborn animals, seeding the lymph nodes and other lymph tissues; it is partly responsible for the development of the organism's immune system (see immunity). The kidney is sometimes considered an endocrine gland because it secretes the hormone renin which, with other substances, regulates blood pressure. The kidney produces a glycoprotein called erythropoietin, which stimulates red blood cell production. The pineal gland produces a substance called melatonin, which helps regulate the body's internal clock.

The Hypothalamus

Physiological processes are under nervous system as well as endocrine control and a gland adjacent to the pituitary, called the hypothalamus, mediates between the two systems. The hypothalamus secretes pituitary-regulating substances in response to nervous system stimuli including smell, taste, pain, and emotions. Thus, stress, cold, heat, and other stimuli release CRF, or adrenocorticotropic hormone-releasing factor, from the hypothalamus, causing ACTH to be produced by the pituitary, which in turn stimulates the production of the adrenal hormone cortisol. Similar chemical regulatory mechanisms operate in the regulation of the sex and thyroid hormones. Hypothalamic activity is also regulated by other body substances, e.g., cortisol inhibits the production of hypothalamic CRF.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"endocrine system." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"endocrine system." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/endocrine-system

"endocrine system." The Columbia Encyclopedia, 6th ed.. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/endocrine-system

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Endocrine Disruptors

ENDOCRINE DISRUPTORS

An endocrine disruptor is any chemical (including dietary) or physical agent that modulates one or several of the endocrine organs or the function of these organs. The endocrine system works primarily as a closed feedback loop that functions to maintain homeostasis. It is made up of a series of organs, the hormones these organs produce and release, and the organs and tissues affected by these hormones. Endocrine disruptors can modulate any segment of the endocrine system.

A simple example of this loop is the axis between the pituitary and the thyroid. The anterior pituitary gland secretes thyroid-stimulating hormone (TSH) in response to signals from the hypothalamus. The TSH travels to the thyroid gland via the circulatory system and activates pathways that lead to uptake of iodine and the synthesis and eventual secretion of thyroid hormone. The thyroid hormone is carried in the blood to end organs and tissues where it regulates metabolism. Thyroid hormone is eventually metabolized in the liver and excreted as a conjugated metabolite. The decreasing levels in the blood initiate a response in the hypothalamus that restarts the cycle in the pituitary gland. If the levels of thyroid hormone are chronically low (hypothyroidism) the anterior pituitary secretes increasing amounts of TSH that, if the thyroid cannot synthesize thyroid hormone (for example, in the absence of dietary iodine), leads to an enlarged thyroid and in some cases goiter. An endocrine disruptor can be any condition (chemical, diet, radiation, stress) that modulates any of these critical steps, or similar steps in any endocrine system.

Several chemical classes have been identified as endocrine disruptors. The compounds that affect the gonadal-pituitary axis are those that modify the metabolism and/or synthesis of estradiol and testosterone, such as the 5[.alpha]-reductase inhibitors that block production of dihydrotestosterone. These inhibitors are drugs, but are also found in herbs such as saw palmetto. Several chemicals are estrogen mimics that bind to the high-affinity estrogen-binding protein called the estrogen receptor (ER). These chemicals include drugs, natural products, and manufactured chemicals. The concern about estrogen mimics is that they may be involved in causing breast cancer, uterine cancer, and developmental defects. In addition, there is a large class of herbal remedies that are marketed for "female" and "male" health. These preparations are physiologically active and clearly modify the target endocrine system. The standard for determining if these compounds are endocrine modulators is to test them in laboratory systems that have been validated as reasonable surrogates for the intact endocrine system. Estrogen mimics that are widespread in the environment are the substituted phenols such as Bisphenol A, and the longer chain nonyl-and octyl-phenols. These are many times weaker than the active human hormone estradiol in classic assays. Several derivatives of the pesticides DDT and methoxychlor are active as estrogens and/or anti-androgens through mechanisms that involve the respective estrogen or androgen receptors. Natural products, such as genistein from soy, exhibit remarkable estrogenic activity.

There are many endocrine-disrupting chemicals in the environment. The question is what impact these agents have, and how they affect public health. In the occupational setting, if high levels of exposure occur, the risk of disease is very high. This was seen during the manufacturing of diethylstilbestrol (DES), the prototypical endocrine disruptor. The effects of environmental levels of estrogen modulators may be masked by dietary factors, natural hormones, or therapeutic agents. However, exposures of select populations, such as PCB exposures in China, may be of such magnitude as to be discernible in the clinical setting.

In the light of public health concern, the best policy is to minimize exposure and at the same time determine the mechanisms of action of each class of chemical involved in disrupting the function of a particular organ system. It appears that for the most part environmental levels of these pollutants may be tolerated in adults (animals and humans) but there are very few data on the effects of these agents in the developing endocrine systems of the fetus or of children.

Michael Gallo

(see also: Pollution; Toxicology )

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Endocrine Disruptors." Encyclopedia of Public Health. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"Endocrine Disruptors." Encyclopedia of Public Health. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/education/encyclopedias-almanacs-transcripts-and-maps/endocrine-disruptors

"Endocrine Disruptors." Encyclopedia of Public Health. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/education/encyclopedias-almanacs-transcripts-and-maps/endocrine-disruptors

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Endocrine Glands

Endocrine glands

Ductless glands which secrete chemical substances called hormones into the bloodstream which control the internal environment not only of each cell and organ, but of the entire body.

The endocrine glandsthe pineal, pituitary, thyroid, parathyroids, thymus, adrenals, pancreas and gonads (ovaries or testes)comprise the endocrine system. The hypothalamus , the gland in the brain which serves as the command center, operates the endocrine system through the pituitary, a pea-sized gland located under it, which directs the work of all the other glands. The thyroid, a gland in the neck, regulates the body's metabolism. The parathyroids, which are attached to the thyroid, control the amount of calcium and phosphate in the bloodstream. The adrenal glands, located near the kidneys, produce adrenaline which arouses the body to respond to stress and emergencies and other hormones active in carbohydrate metabolism. The pancreas secretes insulin which regulates the level of sugar in the bloodstream. The gonads regulate sexual development, ovulation, and growth of sex organs.

Further Reading

The Endocrine System: Miraculous Messengers. New York: Torstar Books, 1985.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Endocrine Glands." Gale Encyclopedia of Psychology. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"Endocrine Glands." Gale Encyclopedia of Psychology. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/endocrine-glands

"Endocrine Glands." Gale Encyclopedia of Psychology. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/endocrine-glands

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

endocrine gland

endocrine gland (ductless gland) Any gland in an animal that manufactures hormones and secretes them directly into the bloodstream to act at distant sites in the body (known as target organs or cells). Endocrine glands tend to control slow long-term activities in the body, such as growth and sexual development. In mammals they include the pituitary, adrenal, thyroid, and parathyroid glands, the ovary and testis, the placenta, and part of the pancreas (see islets of Langerhans). The activity of endocrine glands is controlled by negative feedback, i.e. a rise in output of hormone inhibits a further increase in its production, either directly or indirectly via the target organ or cell. See also neuroendocrine system. Compare exocrine gland.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"endocrine gland." A Dictionary of Biology. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"endocrine gland." A Dictionary of Biology. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/endocrine-gland-0

"endocrine gland." A Dictionary of Biology. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/endocrine-gland-0

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

endocrine

endocrine Internal or hormonal secretion. Endocrine glands are discrete organs (e.g. pituitary, thyroid, adrenal) which discharge hormones into the circulating blood; they are sometimes known as ‘ductless glands’, in contrast to exocrine glands, which pass their secretions through ducts to an external or internal body surface. There are also cells with endocrine function scattered among other types of tissue (e.g. G-cells, secreting gastrin, in the stomach lining) or grouped within an organ which has other functions as well (e.g. insulin-secreting ‘islets’ in the pancreas; sex hormone-secreting cells in the gonads).

Stuart Judge


See hormones.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"endocrine." The Oxford Companion to the Body. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"endocrine." The Oxford Companion to the Body. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/endocrine

"endocrine." The Oxford Companion to the Body. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/endocrine

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

endocrine glands

endocrine glands Ductless glands that produce and secrete hormones, including the thyroid gland (secreting thyroxine and tri‐iodothyronine), pancreas (insulin and glucagon), adrenal glands (adrenaline, noradrenaline, glucocorticoids, mineralocorticoids), ovary and testes (sex steroids).

Some endocrine glands respond directly to chemical changes in the bloodstream; others are controlled by hormones secreted by the pituitary gland, under the control of the hypothalamus.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"endocrine glands." A Dictionary of Food and Nutrition. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"endocrine glands." A Dictionary of Food and Nutrition. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/endocrine-glands

"endocrine glands." A Dictionary of Food and Nutrition. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/endocrine-glands

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

endocrine system

endocrine system Body system made up of all the endocrine (ductless) glands that secrete hormones directly into the bloodstream to control body functions. The endocrine system (together with the nervous system) controls and regulates all body functions. The chief endocrine glands are the pituitary gland (in the brain), the thyroid gland (in the neck), the adrenal gland (in the abdomen), and the sex gland or gonad (in the female abdomen and male testes).

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"endocrine system." World Encyclopedia. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"endocrine system." World Encyclopedia. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/endocrine-system

"endocrine system." World Encyclopedia. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/endocrine-system

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

endocrinology

en·do·cri·nol·o·gy / ˌendəkrəˈnäləjē/ • n. the branch of physiology and medicine concerned with endocrine glands and hormones. DERIVATIVES: en·do·crin·o·log·i·cal / -ˌkrinəˈläjikəl/ adj. en·do·cri·nol·o·gist / -jist/ n.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"endocrinology." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"endocrinology." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/endocrinology

"endocrinology." The Oxford Pocket Dictionary of Current English. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/endocrinology

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

endocrine gland

endocrine gland (ductless gland) (end-oh-kryn) n. a gland that manufactures one or more hormones and secretes them directly into the bloodstream (and not through a duct to the exterior). Endocrine glands include the pituitary, thyroid, parathyroid, and adrenal glands, the ovary and testis, the placenta, and part of the pancreas.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"endocrine gland." A Dictionary of Nursing. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"endocrine gland." A Dictionary of Nursing. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/endocrine-gland

"endocrine gland." A Dictionary of Nursing. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/endocrine-gland

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

endocrine

en·do·crine / ˈendəkrin/ • adj. Physiol. of, relating to, or denoting glands that secrete hormones or other products directly into the blood. • n. an endocrine gland: the pituitary gland is sometimes called the “master gland” of the endocrines.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"endocrine." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"endocrine." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/endocrine-0

"endocrine." The Oxford Pocket Dictionary of Current English. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/endocrine-0

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

endocrine system

endocrine system In vertebrates, the system of ductless glands which secrete into the blood stream hormones which act on a target elsewhere in the body. See also PITUITARY GLAND.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"endocrine system." A Dictionary of Zoology. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"endocrine system." A Dictionary of Zoology. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/endocrine-system

"endocrine system." A Dictionary of Zoology. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/endocrine-system

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

endocrinology

endocrinology (en-doh-kri-nol-ŏji) n. the study of the endocrine glands and the hormones they secrete.
endocrinologist n.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"endocrinology." A Dictionary of Nursing. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"endocrinology." A Dictionary of Nursing. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/endocrinology

"endocrinology." A Dictionary of Nursing. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/endocrinology

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

endocrinology

endocrinology The study of the structure and functions of the endocrine glands and of the hormones they produce.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"endocrinology." A Dictionary of Biology. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"endocrinology." A Dictionary of Biology. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/endocrinology

"endocrinology." A Dictionary of Biology. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/endocrinology

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

endocrine gland

endocrine gland (ductless gland) A gland that produces hormones.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"endocrine gland." A Dictionary of Zoology. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"endocrine gland." A Dictionary of Zoology. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/endocrine-gland

"endocrine gland." A Dictionary of Zoology. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/endocrine-gland

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

endocrine

endocrine •canine • asinine • leonine • saturnine •Antonine • pavonine • rapine •alpine, cisalpine •pitchpine • orpine •lupine, supine •porcupine • vulpine • salamandrine •alexandrine • sapphirine • taurine •endocrine • aventurine • vulturine •colubrine • lacustrine • estuarine •viperine • passerine • catarrhine •intrauterine, uterine •adulterine • riverine • ensign •internecine, V-sign •piscine • porcine • cosine • thylacine •countersign •hircine, ursine •shoeshine • moonshine • sunshine •earthshine •adamantine, Byzantine, elephantine •Tridentine • Levantine • Bechstein •Epstein • amethystine • Rubinstein •Frankenstein • Palestine • Philistine •turpentine • Einstein • Eisenstein •cispontine, transpontine •serotine • infantine • Wittgenstein •Argentine • Palatine •Ballantyne, valentine •eglantine • Hammerstein •clementine • vespertine • serpentine •Florentine •Lichtenstein, Liechtenstein •Constantine • nemertine • Bernstein •hyacinthine, labyrinthine •Jugurthine • grapevine • bovine •Glühwein • cervine • equine

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"endocrine." Oxford Dictionary of Rhymes. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"endocrine." Oxford Dictionary of Rhymes. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/endocrine

"endocrine." Oxford Dictionary of Rhymes. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/endocrine

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Endocrine Disruptors

Endocrine disruptors


In recent years, scientists have proposed that chemicals released into the environment may be disrupting the endocrine system of humans and wildlife . The endocrine system is a network of glands and hormones that regulates many of the body's functions, such as growth, development, behavior, and maturation. The endocrine glands include the pituitary, thyroid, adrenal, thymus, pancreas, and the male and female gonads (testes and ovaries). These glands secrete regulated amounts of hormones into the bloodstream, where they act as chemical messengers as they are carried throughout the body to control and regulate many the body's functions. The hormones bind to specific cell sites called receptors. By binding to the receptors, the hormones trigger various responses in the tissues that contain the receptors.

An endocrine disruptor is an external agent that interferes in some way with the role of the hormones in the body. The agent might disrupt the endocrine system by affecting any of the stages of hormone production and activity, such as preventing the synthesis of a hormone, directly binding to hormone receptors, or interfering with the breakdown of a natural hormone. Disruption in endocrine function during highly sensitive prenatal periods is especially critical, as small changes in endocrine functions may have delayed consequences that may become evident later in adult life or in a subsequent generation. Adverse effects that might be a result of endocrine disruption include the development of cancers, reproductive and developmental effects, neurological effects (effects on behavior, learning and memory, sensory function, and psychomotor development), and immunological effects (immunosuppression, with resulting disease susceptibility).

Exposure to suspected endocrine disruptors may occur through direct contact with the chemicals or through ingestion of contaminated water, food, or air. Suspected endocrine disruptors can enter air or water from chemical and manufacturing processes and through incineration of products. Industrial workers may be exposed in work settings. Documented examples of health effects of humans exposed to endocrine disrupting chemicals include shortened penises in the sons of women exposed to dioxin-contaminated rice oil in China and reduced sperm count in workers exposed to high doses of Kepone in a Virginia pesticide factory. Diethylstilbestrol (DES), a synthetic estrogen, was used in the 1950s and 1960s by pregnant women to prevent miscarriages. Unfortunately it did not prevent miscarriages, but the teenage daughters of women who had taken DES suffered high rates of vaginal cancers, birth defects of the uterus and ovaries, and immune system suppression. These health effects were traced to their mothers' use of DES.

A variety of chemicals, including some pesticides, have been shown to result in endocrine disruption in animal laboratory studies. However, except for the incidences of endocrine disruption due to chemical exposures in the workplace and to the use of DES, causal relationships between exposure to specific environmental agents and adverse health effects in humans due to endocrine disruption have not yet been firmly established.

There is more evidence that the endocrine systems of fish and wildlife have been affected by chemical contamination in their habitats. Groups of animals that have been affected by endocrine disruption include snails, oysters, fish, alligators and other reptiles, and birds, including gulls and eagles. Whether effects on individuals of a particular species impact populations of that organism is difficult to prove. Whether endocrine disruption is confined to specific areas or is more widespread is also not known. In addition, proving that a specific chemical causes a particular endocrine effect is difficult, as animals are exposed to a variety of chemicals and non-chemical stressors. However, some persistent organic chemicals such as DDT (dichlorodiphenyltrichloroethane), PCBs (polychlorinated biphenyls ), dioxin , and some pesticides have been shown to act as endocrine disruptors in the environment. Adverse effects seen that may be caused by endocrine disrupting mechanisms include abnormal thyroid function and development in fish and birds, decreased fertility in shellfish, fish, birds, and mammals, decreased hatching success in fish, birds, and reptiles, demasculinization and feminization of fish, birds, reptiles, and mammals, defeminization and masculinization of gastropods , fish, and birds, and alteration of immune and behavioral function in birds and mammals. Many potential endocrine disrupting chemicals are persistent and bioaccumulate in fatty tissues of organisms and increase in concentration as they move up through the food web. Because of this persistence and mobility, they can accumulate and harm organisms far from their original source.

More information is needed to define the ecological and human health risks of endocrine disrupting chemicals. Epidemiological investigations, exposure assessments, and laboratory testing studies for a wide variety of both naturally occurring and synthetic chemicals are tools that are being used to determine whether these chemicals as environmental contaminants have the potential to disrupt hormonally mediated processes in humans and animals.

[Judith L. Sims ]


RESOURCES

BOOKS


Colburn, Theo, Dianne Dumanoski, and John Peterson Myers.Our Stolen Future: Are We Threatening Our Fertility, Intelligence, and Survival? A Scientific Detective Story. New York, NY: Penguin Books USA, 1997.

Gillette, Louis J., and D. Andrew Crain. ed.Environmental Endocrine Disruptors. London, England: Taylor & Francis Group, 2000.

Krimsky, Sheldon, and Lynn Goldman.Hormonal Chaos: The Scientific and Social Origins of the Environmental Endocrine Hypothesis. Baltimore, MD: Johns Hopkins Press, 1999.

Weyer, Peter, and David Riley.Endocrine Disruptors and Pharmaceuticals in Drinking Water. Denver, CO: American Water Works Association, 2001.

PERIODICALS

Kavlock, Robert J. et al. "Research Needs for the Risk Assessment of Health and Environmental Effects of Endocrine Disruptors: A Report of the U.S. EPA-Sponsored Workshop." Environmental Health Perspectives 104 (1996):
715740.

OTHER

Committee on Environment and Natural Resources, National Science and Technology Council. Endocrine Disruptors Research Initiative. U.S. Environmental Protection Agency, Washington, DC, June 29, 1999. [cited June 1, 2002]. <http://www.epa.gov/endocrine/>.

Endocrine Disruptors. Natural Resources Defense Council, November 25, 1998. [cited June 1, 2002]. <http://nrdc.org/health/effects/qendoc.asp>.

Endocrine Disruptors. World Wildlife Fund, [cited June 1, 2002], <http://www.wildlife.org/toxics/progareas/ed/>.

Technical Panel, Office of Research and Development, Office of Prevention, Pesticides, and Toxic Substances. Special Report on Environmental Endocrine Disruption: An Effects Assessment and Analysis. EPA/630/R-96/012, U.S. Environmental Protection Agency, Washington, DC, 1997.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Endocrine Disruptors." Environmental Encyclopedia. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"Endocrine Disruptors." Environmental Encyclopedia. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/endocrine-disruptors

"Endocrine Disruptors." Environmental Encyclopedia. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/endocrine-disruptors

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Endocrine System

Endocrine System

Definition

The endocrine system is a widespread group of glands and organs that acts as the body's control system for producing, storing, and secreting chemical substances called hormones.

Description

The primary glands that compose the endocrine system are the hypothalamus, pituitary, thyroid, parathyroid, adrenal, pineal, ovary, and testes. The pancreas, considered both an organ and a gland, is also part of the system. The thymus is sometimes considered an endocrine-system organ. Although not part of the endocrine system, other organs that secrete hormones are the heart, brain, lungs, kidneys, liver, skin, and placenta. The word "endocrine" means that in response to specific stimuli, the hormones produced by the glands are released into the bloodstream.

Function

Hormones are compounds produced by the endocrine glands. They generally control the growth, development, and metabolism of the body; the electrolyte composition of body fluids; and reproduction. The specific functions of the endocrine glands and pancreas are unique.

Pituitary gland

The pituitary is the master gland of the endocrine system. Located at the base of the brain, the gland, which is about the size of a marble, consists of two parts: anterior and posterior. The anterior pituitary produces hormones that either stimulate other glands (such as adrenal, testis, ovary, and thyroid) to produce target-gland hormones, or directly affect the target organs.

Three of these hormones—adrenocorticotropic hormone (ACTH), gonadotropins, and thyroid-stimulating hormone (TSH)—act on other glands. ACTH stimulates the adrenal cortex to produce corticosteroid hormones and small amounts of male and female sex hormones. Gonadotropins are two hormones that regulate the production of male and female sex hormones and the egg (ova) and sperm (spermatozoa) cells. TSH stimulates the thyroid gland to produce and release thyroid hormone.

Another pituitary hormone, growth hormone (GH), has a central role in controlling the growth and development of the body and its components, including organs, tissue, and muscle. It also affects the metabolism of carbohydrates, protein, and fat. For example, GH increases glucose levels in the blood by reducing the amount of glucose used by muscle cells and adipose tissue and by promoting glucose production from certain liver molecules. Other functions of GH include increasing the amount of amino acids that cells take from the blood and stimulating the breakdown of lipids (fats) in adipose tissue. The pituitary hormone prolactin acts with other hormones in female breast development and helps regulate breast-milk production (lactation ).

Two hormones, vasopressin and oxytocin, are stored but not produced in the posterior pituitary. Vasopressin, also called arginine vasopressin (AVP), helps the body to conserve water by increasing reabsorption of water from the kidneys. Oxytocin stimulates contractions in the uterus during childbirth and activates milk injection caused by an infant sucking on the breast.

Adrenal glands

The adrenals are small glands on top of the kidneys. The adrenals have two parts: an outer layer called the cortex, and an inner layer called the medulla. The adrenal cortex produces a variety of hormones called corticosteroids, including hydrocortisone (cortisol), which helps increase blood glucose levels. It also reduces the amount of glucose absorbed by muscles and adipose tissue. Another function of cortisol is to protect the body from the adverse affects of stress, including emotional and physical trauma.

The adrenal medulla produces adrenaline and noradrenaline, substances that increase the heart rate and blood pressure during times of stress. Their action is referred to as the "fight-or-flight" response.

Thyroid gland

The thyroid gland is composed of two sections in front of the windpipe and below the voice box. It produces two hormones, thyroxine (T4) and triiodothyronine (T3), which together are called the thyroid hormones. They help regulate growth and development and help in childhood brain development. The thyroid also contains cells that produce the hormone calcitonin, which helps to maintain normal calcium levels in the blood.

Parathyroid glands

The parathyroid complex is composed of four small glands, each the size of a pea, and each located on the four corners of the thyroid gland. They secrete parathyroid hormone, which regulates calcium levels in the blood.

Pancreas

The pancreas is located in the upper abdomen, just behind and below the stomach. It has two functions: to produce various enzymes that aid in digestion; and to produce insulin and glucagon, hormones that are key to the body's management of glucose (sugar) in the blood.

The primary purpose of insulin is to lower blood-glucose levels in the body. It helps form glycogen, proteins, and lipids, which are stored in the body (usually in the liver, muscles, and adipose tissue) to be used for energy. Glucagon increases blood-glucose levels, an action opposite to that of insulin. A strict balance between the glucagon and insulin is required to maintain proper blood-sugar levels.

Hypothalamus

Located deep inside the brain, the hypothalamus maintains direct control of the pituitary gland. It acts as the central "control room" of the endocrine system, directing the activities of the other parts of the system. These activities include regulating eating and drinking, sexual behavior, blood pressure, heart rate, body temperature, emotions, and the sleeping/waking cycle. When the brain receives information indicating that hormonal changes are needed somewhere in the body, the hypothalamus secretes chemicals that stimulate or suppress hormone production in the pituitary gland.

Pineal and thymus

The pineal is located in the center of the brain. This gland secretes melatonin, a hormone that helps regulate the sleeping/waking cycle. Disturbances in the production of melatonin causes jet lag, experienced by many long-distance travelers. Melatonin also influences development of the male and female sex glands. The thymus processes lymphocytes in infants and is partly responsible for immune-system development.

Ovaries and testes

The ovaries and testes, also called the sex glands, produce cells and hormones essential to reproduction and development of the body, including male and female sex characteristics. The three types of sex hormones are estrogens, progestogens, and androgens (including testosterone).

The main role of estrogens is to coordinate development and function of the female genitalia and breasts. Estrogens are also associated with the start of the menstrual bleeding cycle. Estrogen production in the ovaries ceases during menopause. Estrogen is also produced in men (by the testes), though at lower levels than occur in women.

Progestogens are produced in the ovaries during part of the menstrual cycle, and in the placenta during pregnancy. They cause changes in the lining of the uterus to prepare it for pregnancy, and they act with estrogens to stimulate mammary-gland development in the breasts to prepare for lactation. Progesterone is the main progestogen hormone.

The primary androgen produced in the testes is the steroid testosterone. While mainly associated with male development, testosterone is produced in small amounts in women by the ovaries. During pregnancy, testosterone helps to develop the internal and external male sex organs. In males, testosterone promotes the growth of the sex organs and develops or stimulates male characteristics, such as deepening voice; growth of facial, pubic, and other body hair; and muscle growth and strength. In adult males, testosterone maintains the masculine characteristics and sexual potency and regulates sperm production.

Role in human health

A wide variety and dozens of symptoms can indicate a hormonal imbalance in the body. However, a specific group of symptoms give an initial indication of a problem in the endocrine system. For example, excessive thirst, frequent urination, and unexplained weight loss are classic signs of diabetes mellitus, the most common endocrine disorder. Many primary care physicians still treat endocrine problems, especially diabetes, themselves. However, the primary care doctor often makes a preliminary diagnosis and then refers the patient to an endocrine-system specialist, called an endocrinologist.

Disorders of the endocrine system often, but not always, result from an over- or underproduction of a particular hormone. Too much or too little of a hormone can be harmful. The endocrine organs use a feedback mechanism to regulate hormone levels. It acts much like a household thermostat, increasing production of a specific hormone when it detects too little in the blood, or decreasing production when it detects too much or the right amount. Tight control of hormone levels is needed for the body to function properly. The endocrine organs secrete hormones directly into the bloodstream, where special proteins usually bind to them, helping to maintain them as they travel through the body.

KEY TERMS

Adipose tissue— Connective tissue in which fat is stored and that has the cells distended by droplets of fat.

Autoimmune— A term that refers to a condition in which antibodies or T cells attack the molecules, cells, or tissue of the body organ or system producing them.

Electrolyte— A nonmetallic electric conductor in which current is carried by ion movement.

Lactation— The secretion of milk by the mammary gland in the breasts.

Lymphocytes— Weak cells produced in the lymphoid tissue.

Menopause— The period when natural menstruation stops, usually between ages of 45 and 50.

Menstruation— The discharge of blood from the uterus that occurs in approximately monthly intervals in females, starting at puberty.

Common diseases and disorders

There are two basic classes of endocrine disorders: problems associated with hormone-production levels, and problems caused by tissues that are unable to respond to hormones. Hormone-production disorders are broken into two groups: insufficient hormone production, called hypofunction; and too much hormone production, called hyperproduction. Endocrine-system disorders include the following:

  • Diabetes mellitus is a disease that includes type 1 and type 2 diabetes. Type 1 is an autoimmune disease caused when the immune system destroys certain insulin-producing cells in the pancreas. This causes the pancreas to produce little or no insulin. Type 1 diabetes usually develops in children and young adults, but it can appear at any age. Symptoms include increased thirst and urination, unexplained weight loss, blurred vision, and extreme fatigue. There is no cure; insulin, first used in 1921, remains the only treatment. Type 2 diabetes accounts for 90-95% of diabetes cases. It usually develops in adults over age 40 and is usually associated with obesity. In type 2, the pancreas produces insulin, but the hormone is not used effectively by the body, a condition called insulin resistance. Several years after onset, insulin production decreases below the level needed to maintain glucose homeostasis. The result is the same as for type 1 diabetes: glucose builds up in the blood because the body cannot use it efficiently. Symptoms develop gradually and include increased thirst and urination, weight loss, fatigue, nausea, blurred vision, frequent infections, and wounds or sores that heal slowly. Insulin resistance is treated with drugs such as thiazolidinedions (rosiglitazone and pioglitazone) and biguanides (metformen). When insufficient insulin is produced, type 2 diabetes appears. However, research indicates progression of insulin resistance to type 2 diabetes can usually be halted or slowed with the insulinresistance medications, or by lifestyle changes that result in weight loss.
  • Hypothyroidism is caused by the thyroid gland producing too little thyroid hormone. It can lead to severe hypothyroidism, a disorder that usually develops after age 40. Symptoms include intolerance to cold, lethargy, fatigue, weight gain, and mental sluggishness. Congenital hypothyroidism is present at birth and has the same symptoms. If left untreated, it can lead to mental retardation. The standard treatment for both hypothyroid disorders is thyroid hormone-replacement medications such as levothyroxine (Synthroid, Unithroid, Levoxyl, and Levothroid) and tri-iodothyrine.
  • Hyperthyroidism is due to an excess of thyroid hormones and affects women more frequently than men. Symptoms include nervousness, weight loss, intolerance to heat, diarrhea, heart palpitations, and insomnia. Some patients experience protruding eyes and trembling. Treatments include medications to inhibit thyroid-hormone production, and removal or destruction of the thyroid gland with radioactive iodine. The most common cause of the excessive thyroid production is Graves' disease, an autoimmune disorder of the thyroid gland.
  • Addison's disease is caused by underactivity or immune-system destruction of the adrenal gland. It can be life-threatening if left untreated. Symptoms include weakness, fatigue, nausea, dehydration, fever, and hyperpigmentation (darkening of the skin without sun exposure.) The standard treatment is with corticosteroid hormones and adequate dietary salt.
  • Cushing's syndrome and Cushing's disease are different disorders with similar symptoms: obesity, weakness, easily bruised skin, acne, and hypertension (high blood pressure.) Cushing's syndrome is usually caused by excessive production of glucocorticoid hormones in the adrenal gland. However, it can sometimes be caused by benign or cancerous tumors of the adrenal gland. Cushing's disease usually results from the overproduction of the adrenocoticotropic hormone in the pituitary gland, due to a benign tumor. Treatment for both disorders can include surgery, radiation therapy, chemotherapy, and blocking production of the glucorticoid hormones with drugs.
  • Less common endocrine disorders include acromegaly, gigantism, and hypogonadism. Acromegaly occurs in adults and gigantism in children. Both are caused by a pituitary tumor that spurs overproduction of growth hormone. Hypogonadism causes delayed sexual maturity in children and infertility in adults. It is caused by underproduction of folliclestimulating hormone (FSH) in the pituitary gland.

Resources

BOOKS

Constanti, A., et al. Basic Endocrinology for Students of Pharmacy and Allied Health Sciences. Newark, NJ: Harwood Academic, 1998.

Hall, J. E., and L. Nieman, editors. Handbook of Diagnostic Endocrinology. Totowa, NJ: Humana Press, 2003.

Krentz, A. Churchill's Pocket Book of Diabetes. New York: Churchill Livingston, 2001.

Matthew, N. J. How the Endocrine System Works. Malden, MA: Blackwell Science, 2001.

Wilson, J. D., et al. Williams Textbook of Endocrinology. St. Louis, MO: W. B. Saunders, 2002.

PERIODICALS

American Diabetes Association. "Implications of the Diabetes Control and Complications Study." Diabetes Care (January 2001): 825.

Elliott, B. "Diagnosing and Treating Hypothyroidism." The Nurse Practitioner (March 2000): 92+.

Emanuele, N. and M. A. "The Endocrine System: Alcohol Alters Critical Hormonal Balance." Alcohol Health and Research World (Winter 1997): 53-64.

Hiller-Sturmhofel, S., and A. Bartke. "The Endocrine System: An Overview." Alcohol Health and Research World (Summer 1998): 153-154.

Lamberts, S. W. J., et al. "The Endocrinology of Aging." Science (October 17, 1997): 419-424.

Wilson, J. D. "Prospects for Research for Disorders of the Endocrine System." The Journal of the American Medical Association (February 7, 2001): 624.

ORGANIZATIONS

American Association of Clinical Endocrinologists. 1000 Riverside Avenue, Suite 205, Jacksonville, FL 32204. (904) 353-7878. 〈http://www.aace.com〉.

American Diabetes Association. 1701 North Beauregard Street, Alexandria, VA 22311. (800) 342-2383. 〈http://www.diabetes.org〉.

American Thyroid Association. Montefiore Medical Center, 111 East 210th Street, Room 311, Bronx, NY 10467. (718) 882-6085. 〈http://www.thyroid.org〉.

Endocrine Society. 4350 East West Highway, Suite 500, Bethesda, MD 20814. (301) 941-0200. 〈http://www.endo-society.org〉.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Endocrine System." Gale Encyclopedia of Nursing and Allied Health. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"Endocrine System." Gale Encyclopedia of Nursing and Allied Health. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/endocrine-system

"Endocrine System." Gale Encyclopedia of Nursing and Allied Health. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/endocrine-system

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Endocrine System

Endocrine System

History of endocrinology

Basic endocrine principles

The pituitary

The pineal

The thyroid

The parathyroids

The thymus

The pancreas

The adrenals

The ovaries

The testes

Endocrine disorders

Resources

The endocrine system is the human bodys group of specialized organs and tissues that produce, store, and secrete chemical hormones. This network of nine glands and over 100 hormones that maintain and regulate numerous events throughout the body. The glands of the endocrine system include the pituitary, thyroid, parathyroids, thymus, pancreas, pineal, adrenals, and ovaries or testes: in addition, the hypothalamus, in the brain, regulates the release of pituitary hormones. Each of these glands secrete hormones (chemical messengers) into the blood stream. Once hormones enter the blood, they travel throughout the body and are detected by receptors that recognize

specific hormones. These receptors exist on target cells and organs. Once a target site is bound by a particular hormone, a cascade of cellular events follows that culminates in the physiological response to a particular hormone.

The endocrine system differs from the exocrine system in that exocrine glands contain ducts that direct their hormones to specific sites; whereas endocrine hormones travel through blood until they reach their destination. The endocrine is also similar to the nervous system, because both systems regulate body events and communicate through chemical messengers with target cells. However, the nervous system transmits neurotransmitters (also chemical messengers) between neighboring neurons via nerve extension, and neurotransmitters do not generally enter the circulation. Yet, some overlap between hormones and neurotransmitters exists, which gives rise to chemical signals called neurohormones that function as part of the neuroendocrine system. The endocrine system oversees many critical life processes involving metabolism, growth, reproduction, immunity, and homeostasis. The branch of medicine that studies endocrine glands and the hormones that they secrete is called endocrinology.

History of endocrinology

Although some ancient cultures noted biological observations grounded in endocrine function, modern understanding of endocrine glands and how they secrete hormones has evolved only in the last 300 years. Ancient Egyptian and Chinese civilizations castrated (removed the testicles of) a servile class of men called eunuchs. It was noted that eunuchs were less aggressive than other men, but the link of this behavior to testosterone was not made until recently.

Light was shed on endocrine function during the seventeenth and eighteenth centuries by a few significant advances. Seventeenth century English scientist, Thomas Wharton (16141673) noted the distinction between ductile and ductless glands. In the 1690s, Dutch scientist Fredrik Ruysch (16381731) first stated that the thyroid secreted important substances into the blood stream. A few decades later, Theophile Bordeu (17221776) claimed that emanations were given off by some body parts that influenced functions of other body parts.

One of the greatest early experiments performed in endocrinology was published by A. A. Berthold (18031861) in 1849. Berthold took six young male chickens, and castrated four of them. The other two chickens were left to develop normally; thus, they were used comparatively as control samples. Two of the castrated chickens were left to become chicken eunuchs. However, what Berthold did with the other two castrated chickens is what really changed endocrinology. He transplanted the testes back into these two chickens at a distant site from where they were originally. The two castrated chickens never matured into roosters with adult combs or feathers. However, the chickens that received transplanted testes did mature into normal adult roosters. This experiment revealed that hormones that could access the blood stream from any site would function correctly in the body and that hormones did, in fact, travel freely in the circulation.

The same year Berthold published his findings, Thomas Addison (17931860), a British scientist, reported one of the first well documented endocrine diseases which was later named Addisons disease (AD). AD patients all had a gray complexion with sickly skin; they also had weak hearts and insufficient blood levels of hemoglobin necessary for oxygen transport throughout the body. On autopsy, each of the patients Addison studied was found to have diseased adrenal glands. This disease can be controlled today if it is detected early. President John F. Kennedy (19171963) suffered from AD.

Basic endocrine principles

Most endocrine hormones are maintained at specific concentrations in the plasma, the non-cellular, liquid portion of the blood. Receptors at set locations monitor plasma hormonal levels and inform the gland responsible for producing that hormone if levels are too high or too low for a particular time of day, month, or other life period. When excess hormone is present, a negative feedback loop is initiated such that further hormone production is inhibited. Most hormones have this type of regulatory control. However, a few hormones operate on a positive feedback cycle such that high levels of the particular hormone will activate release of another hormone. With this type of feedback loop, the end result is usually that the second hormone released will eventually decrease the initial hormones secretion. An example of positive feedback regulation occurs in the female menstrual cycle, where high levels of estrogen stimulate release of the pituitary hormone, luteinizing hormone (LH).

All hormones are influenced by numerous factors. The hypothalamus can release inhibitory or stimulatory hormones that determine pituitary function. In addition, every physiological component that enters the circulation can affect some endocrine function. Overall, this system uses multiple bits of chemical information to hormonally maintain a biochemically balanced organism.

Endocrine hormones do not fall into any one chemical class, but most are either a protein (polypep-tides, peptides, and glycoproteins are also included in this category) or a steroid. Protein hormones bind cell-surface receptors and activate intracellular events that carry out the hormones response. Steroid hormones, on the other hand, usually travel directly into the cell and bind a receptor in the cells cytoplasm or nucleus. From there, steroid hormones (bound to their receptors) interact directly with genes in the DNA (deoxy-ribonucleic acid) to elicit a hormonal response.

The pituitary

The pituitary gland has long been called the master gland, because it secretes multiple hormones that, in turn, trigger the release of other hormones from other endocrine sites. The pituitary is roughly situated behind the nose and is anatomically separated into two distinct lobes, the anterior pituitary (AP) and the posterior pituitary (PP). The entire pituitary hangs by a thin piece of tissue, called the pituitary stalk, beneath the hypothalamus in the brain. The AP and PP are sometimes called the adenohypophysis and neurohypophysis, respectively.

The PP secretes two hormones, oxytocin and anti-diuretic hormone (ADH), under direction from the hypothalamus. Direct innervation of the PP occurs from regions of the hypothalamus called the supraoptic and paraventricular nuclei. Although the PP secretes its hormones into the bloodstream through blood vessels that supply it, it is regulated in a neuroendocrine fashion. The AP, on the other hand, receives hormonal signals from the blood supply within the pituitary stalk.

AP cells are categorized according to the hormones that they secrete. The hormone-producing cells of the AP include: somatotrophs, corticotrophs, thyrotrophs, lactotrophs, and gonadotrophs. Somatotrophs secrete growth hormone; corticotrophs secrete adrenocortico-tropic hormone (ACTH); thyrotrophs secrete thyroid stimulating hormone (TSH); lactotrophs secrete prolactin; and gonadotrophs secrete LH and follicle stimulatory hormone (FSH). Each of these hormones sequentially signals a response at a target site. While ACTH, TSH, LH, and FSH primarily stimulate other major endocrine glands, growth hormone and prolactin primarily coordinate an endocrine response directly on bones and mammary tissue, respectively.

The pineal

The pineal gland is a small cone-shaped gland believed to function as a body clock. The pineal is located deep in the brain just below the rear-most portion of the corpus callosum (a thick stretch of nerves that connects the two sides of the brain). The pineal gland, also called the pineal body, has mystified scientists for centuries. Seventeenth century French mathematician and philosopher Rene´ Descartes (15961650) speculated that the pineal was the seat of the soul. However, its real function is somewhat less grandiose than that described by Descartes.

The pineal secretes the hormone melatonin, which fluctuates on a daily basis with levels highest at night. Although its role is not well understood, some scientists believe that melatonin helps to regulate other diurnal events, because melatonin fluctuates in a 24-hour period. Exactly what controls melatonin levels is not well understood either; however, visual registration of light may regulate the cycle.

The thyroid

The thyroid is a butterfly-shaped gland that wraps around the back of the esophagus. The two lobes of the thyroid are connected by a band of tissue called the isthmus. An external covering of connective tissue separates each lobe into another 20 to 40 follicles. Between the follicles are numerous blood and lymph vessels in another connective tissue called stroma. The epithelial cells around the edge of the follicles produce the major thyroid hormones.

The major hormones produced by the thyroid are triiodothyronine (T3), thyroxine(T4), and calcitonin. T3 and T4 are iodine-rich molecules that fuel metabolism. The thyroid hormones play several important roles in growth, metabolism, and development. The thyroid of pregnant women often become enlarged in late pregnancy to accommodate metabolic requirements of both the woman and the fetus.

Thyroid hormones accelerate metabolism in several ways. They promote normal growth of bones and increase growth hormone output. They increase the rate of lipid synthesis and mobilization. They increase cardiac output by increasing rate and strength of heart contractions. They can increase respiration, the number of red blood cells in the circulation, and the amount of oxygen carried in the blood. In addition, they promote normal nervous system development including nerve branching.

The parathyroids

While most people have four small parathyroid glands poised around the thyroid gland, about 14% of the population have one or two additional parathyroid glands. Because these oval glands are so small, the extra space occupied by extra glands does not seem to be a problem. The sole function of these glands is to regulate calcium levels in the body. Although this may seem like a simple task, the maintenance of specific calcium levels is critical. Calcium has numerous important bodily functions. Calcium makes up 2 to 3% of adult weight with roughly 99% of the calcium in bones. Calcium also plays a pivotal role in muscle contraction and neurotransmitter secretion.

The thymus

In young children, the thymus extends into the neck and the chest, but after puberty, it begins to shrink. The size of the thymus in most adults is very small. Like some other endocrine glands, the thymus has two lobes connected by a stalk. The thymus secretes several hormones that promote the maturation of different cells of the immune system in young children. In addition, the thymus oversees the development and learning of a particular type of immune system cell called a T lymphocyte, or T cell.

Although many details of thymal hormonal activity are not clear, at least four thymal products have been identified that control T cell and B cell (antibody-producing immune cells) maturation. The four products are: thymosin, thymic humoral factor (THF), thymic factor (TF), and thymopoietin. Because the viral disease, AIDS (acquired immune deficiency syndrome), is characterized by T cell depletion, some AIDS treatment approaches have tried administering tymosin to boost T cell production.

The pancreas

The pancreas is a large endocrine and exocrine gland situated below and behind the stomach in the lower abdomen. The pancreas is horizontally placed such that its larger end falls to the right and its narrower end to the left. Clusters of exocrine pancreatic cells called acini secrete digestive enzymes into the stomach; while endocrine cells secrete hormones responsible for maintaining blood glucose levels.

The endocrine cells of the pancreas are contained in the islets of Langerhans which are themselves embedded in a rich network of blood and lymph vessels. About one million islet cells make up about 2% of the total pancreas: the islet cells are quite small. The three major types of endocrine cells within the islets are alpha cells, beta cells, and delta cells. Beta cells make up about 70% of islet cells and secrete insulin.

Alpha cells, which secrete glucagon, comprise roughly 20% of islet cells. Delta cells, which comprise up to 8% of islet cells, secrete somatostatin. Another pancreatic hormone, called pancreatic polypeptide, is secreted by F cells in the islets and has just recently been characterized.

Insulin is secreted in response to high plasma glucose levels. Insulin facilitates glucose uptake into blood cells; thus, reducing plasma glucose levels. Glucagon has the opposite effect; low plasma glucose triggers the breakdown of stored glucogen in the liver and glucose release into the blood. By balancing these two hormones, the islets continually regulate circulating glucose levels. Both hormones are contained within secretory vesicles in the cells that release them. In addition, the monitoring of blood glucose concentrations are evaluated directly at the pancreas, as opposed to being mediated by another gland such as the pituitary. Moreover, nerve endings at the islets contribute in regulating insulin and glucagon secretion. The hormone somatotostatin is also released under conditions of increased blood glucose, glucagon, and amino acids. Somatostatin inhibits additional insulin release.

The overall effect of insulin is to store fuel in muscle and adipose tissue. It also promotes protein synthesis. Hence, insulin is said to be anabolic, which means that it works to build up instead of break down energy-storing molecules. Glucagon, however, is metabolic, since it promotes glycogen breakdown. However, each of the pancreatic hormones is also considered to be paracrine, since they also modulate other pancreatic cells.

Diabetes mellitus (DM) is a very serious endocrine disorder caused by insulin dysfunctions. In type I DM, the beta cells are defective and cannot produce enough insulin. Type II DM is caused by a lack of target cell-receptor responsiveness to available insulin. While type I requires regular insulin injections, type II can be controlled with diet.

The adrenals

One of the two adrenals sit atop each kidney and are divided into two distinct regions, the cortex and the medulla. The outer area makes up about 80% of each adrenal and is called the cortex. In addition, the inner portion is called the medulla. The adrenals provide the body with important buffers against stress while helping it adapt to stressful situations.

The cells of the cortex form three distinct layers, the zona glomerulosa (ZG), the zona fasciculata (ZF), and the zona reticularis (ZR). All three layers secrete steroid hormones. The ZG secretes mineralocorticoids; the ZF secretes glucocorticoids; and the ZR secretes primary androgens. Cholesterol droplets are interspersed throughout the cortex for synthesis of cortical steroids. ACTH released from the anterior pituitary triggers glucocorticoids release from the ZF. The major glucocorticoid in humans is cortisol, which fluctuates in concentration throughout the day: the highest levels exist in the morning around 8 A.M. with the lowest levels around midnight. Both physical and emotional stress can affect cortisol secretion. The major human mileralocorticoid is aldosterone. Aldosterone acts on the kidneys and is important in regulating fluid balance. It triggers increased extracellular fluid that leads to increased blood pressure.

Cells of the adrenal medulla, called chromaffin cells, secrete the hormones epinephrine (adrenaline) and non-epinephrine (nor-adrenaline). Chromaffin cells are neuroendocrine cells that function like some nerve fibers of the sympathetic nervous system. However, these cells are endocrine because the neuro-hormones that they release target distant organs. Although the effects of these two medullary hormones are the same whether they originate in the endocrine or the nervous system, endocrine hormonal effects are prolonged because they are removed more slowly from blood than from a nerve terminal. Both cortical and medullary hormones work together in emergencies, or stressful situations, to meet the physical demands of the moment.

The ovaries

The ovaries are located at the end of each fallopian tube in the female reproductive tract, and they produce the female reproductive hormones: estrogen, progesterone, and relaxin. Although the fluctuation of these hormones is critical to the female menstrual cycle, a hormone from the hypothalamus, called a releasing factor, initially triggers them. They enable gonadotrophs in the pituitary to release LH and FSH that, in turn, regulate part of the menstrual cycle. All of these hormones work together as part of the endocrine system to ensure fertility. They are also important for the development of sexual characteristics during puberty.

Each month after puberty, females release a single egg (ovulation) when the pituitary releases LH. Prior to ovulation, the maturing egg releases increasing levels of estrogen that inform the pituitary to secrete LH. While an egg travels down the fallopian tube, progesterone is released that prevents another egg from beginning to mature. Once an egg is shed in the uterine lining (in menstruation) the cycle can begin again. During pregnancy, high levels of circulating estrogen and progesterone prevent another egg from maturing. Estrogen levels fall dramatically at menopause, signifying the end of menstrual cycling and fertility. Menopause usually occurs between the ages of 40 and 50 years.

Endocrine regulation of the female reproductive tract does not stop during pregnancy. In fact, more sex hormones are released during pregnancy than during any other phase of female life. At this time, a new set of conditions support and protect the developing baby. Even cells of the developing embryo begin to release some hormones that keep the uterine lining intact for the first couple of months of pregnancy. High progesterone levels prevent the uterus from contracting so that the embryo is not disturbed. Progesterone also helps to prepare breasts for lactation. Towards the end of pregnancy, high estrogen levels stimulate the pituitary to release the hormone, oxytocin, which triggers uterine contractions. Prior to delivery, the ovaries release the hormone, relaxin, a protein which causes the pelvic ligaments to become loose for labor.

The testes

The two testes are located in the scrotum, which hangs between the legs behind the penis. The testes is devoted primarily to sperm production, but the remaining cells, called Leydig cells, produce testosterone. Testosterone caries out two important endocrine tasks in males: it facilitates sexual maturation, and it enables sperm to mature to a reproductively competent form. Healthy men remain capable of fertilizing an egg throughout their post-pubertal life. However, testosterone levels do show a gradual decline after about the age of 40 years with a total drop of around 20% by age 80 years.

Testosterone also has the important endocrine function of providing sexual desire in both men and women. Although reproduction can occur without maximal desire, this added incentive increases the likelihood of reproduction. Human sexual behavior is also influenced by several other factors including thoughts and beliefs.

Endocrine disorders

As much as 10% of the population will experience some endocrine disorder in their lifetime. Most endocrine disorders are caused by a heightened or diminished level of particular hormones. For example, excess growth hormone can cause giantism (unusually large stature). Tumors in endocrine glands are one of the major causes of hormone overproduction.

KEY TERMS

Exocrine Glands with ducts that direct hormones to a specific target.

Hormone underproduction is often due to a mutation in the gene that codes for a particular hormone or the hormones receptor; even if the hormone is normal, a defective receptor can render the hormone ineffective in eliciting a response. The distinction between the pancreatic endocrine disorders, diabetes mellitus types I and II, make this point very clearly. In addition, underproduction of growth hormones can lead to dwarfism. Insufficient calcitonin from the thyroid can also lead to cretinism that is also characterized by diminished stature due to low calcium availability for bone growth.

The importance of diet can not be overlooked in some endocrine disorders. For example, insufficient dietary iodine (required for T3 and T4 synthesis) can cause goiters. Goiters are enlarged thyroid glands caused by the thyroids attempt to compensate for low iodine with increased size. Certain endocrine imbalances can also cause mental problems ranging from poor ability to concentrate to mental retardation (as in cretinism).

See also Chemoreception.

Resources

BOOKS

Germann, William J. Principles of Human Physiology. San Francisco, CA: Pearson Benjamin Cummings, 2005.

Greenstein, Ben. The Endocrine System at a Glance. Malden, MA: Blackwell Publishing, 2006.

Rushton, Lynette. The Endocrine System. Philadelphia, PA: Chelsea House Publishers, 2004.

Van De Graaff, Kent M., and R. Ward Rhees, eds. Human Anatomy and Physiology: Based on Schaums Outline of Theory and Problems of Human Anatomy and Physiology. New York: McGraw-Hill, 2001.

Louise Dickerson

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Endocrine System." The Gale Encyclopedia of Science. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"Endocrine System." The Gale Encyclopedia of Science. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/endocrine-system

"Endocrine System." The Gale Encyclopedia of Science. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/endocrine-system

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Endocrine System

Endocrine system

Definition

The endocrine system is a widespread group of glands and organs that acts as the body's control system for producing, storing, and secreting chemical substances called hormones.

Description

The primary glands that compose the endocrine system are the hypothalamus, pituitary, thyroid, parathyroid, adrenal, pineal, ovary, and testes. The pancreas , considered both an organ and a gland, is also part of the system. The thymus is sometimes considered an endocrine-system organ. Although not part of the endocrine system, other organs that secrete hormones are the heart , brain , lungs , kidneys , liver , skin, and placenta. The word "endocrine" means that in response to specific stimuli, the hormones produced by the glands are released into the bloodstream.

Function

Hormones are compounds produced by the endocrine glands. They generally control the growth, development, and metabolism of the body; the electrolyte composition of body fluids; and reproduction. The specific functions of the endocrine glands and pancreas are unique.

Pituitary gland

The pituitary is the master gland of the endocrine system. Located at the base of the brain, the gland, which is about the size of a marble, consists of two parts: anterior and posterior. The anterior pituitary produces hormones that either stimulate other glands (such as adrenal, testis, ovary, and thyroid) to produce target-gland hormones, or directly affect the target organs.

Three of these hormones—adrenocorticotropic hormone (ACTH), gonadotropins, and thyroid-stimulating hormone (TSH)—act on other glands. ACTH stimulates the adrenal cortex to produce corticosteroid hormones and small amounts of male and female sex hormones. Gonadotropins are two hormones that regulate the production of male and female sex hormones and the egg (ova) and sperm (spermatozoa) cells. TSH stimulates the thyroid gland to produce and release thyroid hormone.

Another pituitary hormone, growth hormone (GH), has a central role in controlling the growth and development of the body and its components, including organs, tissue, and muscle. It also affects the metabolism of carbohydrates ,

protein, and fat. For example, GH increases glucose levels in the blood by reducing the amount of glucose used by muscle cells and adipose tissue and by promoting glucose production from certain liver molecules. Other functions of GH include increasing the amount of amino acids that cells take from the blood and stimulating the breakdown of lipids (fats ) in adipose tissue. The pituitary hormone prolactin acts with other hormones in female breast development and helps regulate breast-milk production (lactation ).

Two hormones, vasopressin and oxytocin, are stored but not produced in the posterior pituitary. Vasopressin, also called arginine vasopressin (AVP), helps the body to conserve water by increasing reabsorption of water from the kidneys. Oxytocin stimulates contractions in the uterus during childbirth and activates milk injection caused by an infant sucking on the breast.

Adrenal glands

The adrenals are small glands on top of the kidneys. The adrenals have two parts: an outer layer called the cortex, and an inner layer called the medulla. The adrenal cortex produces a variety of hormones called corticosteroids , including hydrocortisone (cortisol), which helps increase blood glucose levels. It also reduces the amount of glucose absorbed by muscles and adipose tissue. Another function of cortisol is to protect the body from the adverse affects of stress , including emotional and physical trauma.

The adrenal medulla produces adrenaline and noradrenaline, substances that increase the heart rate and blood pressure during times of stress. Their action is referred to as the "fight-or-flight" response.

Thyroid gland

The thyroid gland is composed of two sections in front of the windpipe and below the voice box. It produces two hormones, thyroxine (T4) and tri-iodothyronine (T3), which together are called the thyroid hormones. They help regulate growth and development and help in childhood brain development. The thyroid also contains cells that produce the hormone calcitonin, which helps to maintain normal calcium levels in the blood.

Parathyroid glands

The parathyroid complex is composed of four small glands, each the size of a pea, and each located on the four corners of the thyroid gland. They secrete parathyroid hormone, which regulates calcium levels in the blood.

Pancreas

The pancreas is located in the upper abdomen, just behind and below the stomach . It has two functions: to produce various enzymes that aid in digestion; and to produce insulin and glucagon, hormones that are key to the body's management of glucose (sugar) in the blood.

The primary purpose of insulin is to lower blood-glucose levels in the body. It helps form glycogen, proteins , and lipids, which are stored in the body (usually in the liver, muscles, and adipose tissue) to be used for energy. Glucagon increases blood-glucose levels, an action opposite to that of insulin. A strict balance between the glucagon and insulin is required to maintain proper blood-sugar levels.

Hypothalamus

Located deep inside the brain, the hypothalamus maintains direct control of the pituitary gland . It acts as the central "control room" of the endocrine system, directing the activities of the other parts of the system. These activities include regulating eating and drinking, sexual behavior, blood pressure, heart rate, body temperature, emotions, and the sleeping/waking cycle. When the brain receives information indicating that hormonal changes are needed somewhere in the body, the hypothalamus secretes chemicals that stimulate or suppress hormone production in the pituitary gland.

Pineal and thymus

The pineal is located in the center of the brain. This gland secretes melatonin, a hormone that helps regulate the sleeping/waking cycle. Disturbances in the production of melatonin causes jet lag, experienced by many long-distance travelers. Melatonin also influences development of the male and female sex glands. The thymus processes lymphocytes in infants and is partly responsible for immune-system development.

Ovaries and testes

The ovaries and testes, also called the sex glands, produce cells and hormones essential to reproduction and development of the body, including male and female sex characteristics. The three types of sex hormones are estrogens, progestogens, and androgens (including testosterone).

The main role of estrogens is to coordinate development and function of the female genitalia and breasts. Estrogens are also associated with the start of the menstrual bleeding cycle. Estrogen production in the ovary ceases during menopause . Estrogen is also produced in men (by the testes), though at lower levels than occur in women.

Progestogens are produced in the ovaries during part of the menstrual cycle, and in the placenta during pregnancy . They cause changes in the lining of the uterus to prepare it for pregnancy, and they act with estrogens to stimulate mammary-gland development in the breasts to prepare for lactation. Progesterone is the main progestogen hormone.

The primary androgen produced in the testes is the steroid testosterone. While mainly associated with male development, testosterone is produced in small amounts in women by the ovaries. During pregnancy, testosterone helps to develop the internal and external male sex organs. In males, testosterone promotes the growth of the sex organs and develops or stimulates male characteristics, such as deepening voice; growth of facial, pubic, and other body hair; and muscle growth and strength. In adult males, testosterone maintains the masculine characteristics and sexual potency and regulates sperm production.

Role in human health

A wide variety and dozens of symptoms can indicate a hormonal imbalance in the body. However, a specific group of symptoms give an initial indication of a problem in the endocrine system. For example, excessive thirst, frequent urination, and unexplained weight loss are classic signs of diabetes mellitus , the most common endocrine disorder. Many primary-care physicians still treat endocrine problems, especially diabetes, themselves. However, the primary care doctor often makes a preliminary diagnosis and then refers the patient to an endocrine-system specialist, called an endocrinologist.

Disorders of the endocrine system often, but not always, result from an over-or underproduction of a particular hormone. Too much or too little of a hormone can be harmful. The endocrine organs use a feedback mechanism to regulate hormone levels. It acts much like a household thermostat, increasing production of a specific hormone when it detects too little in the blood, or decreasing production when it detects too much or the right amount. Tight control of hormone levels is needed for the body to function properly. The endocrine organs secrete hormones directly into the bloodstream, where special proteins usually bind to them, helping to maintain them as they travel through the body.

Common diseases and disorders

There are two basic classes of endocrine disorders: problems associated with hormone-production levels, and problems caused by tissues that are unable to respond to hormones. Hormone-production disorders are broken into two groups: insufficient hormone production, called hypofunction; and too much hormone production, called hyperproduction. Endocrine-system disorders include the following:

  • Diabetes mellitus is a disease that includes type 1 and type 2 diabetes. Type 1 is an autoimmune disease caused when the immune system destroys certain insulin-producing cells in the pancreas. This causes the pancreas to produce little or no insulin. Type 1 diabetes usually develops in children and young adults, but it can appear at any age. Symptoms include increased thirst and urination, unexplained weight loss, blurred vision , and extreme fatigue. There is no cure; insulin, first used in 1921, remains the only treatment. Type 2 diabetes accounts for 90% to 95% of diabetes cases. It usually develops in adults over age 40 and is usually associated with obesity . In type 2, the pancreas produces insulin, but the hormone is not used effectively by the body, a condition called insulin resistance. Several years after onset, insulin production decreases below the level needed to maintain glucose homeostasis. The result is the same as for type 1 diabetes: glucose builds up in the blood because the body cannot use it efficiently. Symptoms develop gradually and include increased thirst and urination, weight loss, fatigue, nausea, blurred vision, frequent infections, and wounds or sores that heal slowly. Insulin resistance is treated with drugs such as thiazolidinedions (rosiglitazone and pioglitazone) and biguanides (metformen). When insufficient insulin is produced, type 2 diabetes appears. However, research indicates progression of insulin resistance to type 2 diabetes can usually be halted or slowed with the insulin-resistance medications, or by lifestyle changes that result in weight loss.
  • Hypothyroidism is caused by the thyroid gland producing too little thyroid hormone. It can lead to severe hypothyroidism, a disorder that usually develops after age 40. Symptoms include intolerance to cold, lethargy, fatigue, weight gain, and mental sluggishness. Congenital hypothyroidism is present at birth and has the same symptoms. If left untreated, it can lead to mental retardation. The standard treatment for both hypothyroid disorders is thyroid hormone-replacement medications such as levothyroxine (Synthroid, Unithroid, Levoxyl, and Levothroid) and triiodothyrine.
  • Hyperthyroidism is due to an excess of thyroid hormones and affects women more frequently than men. Symptoms include nervousness, weight loss, intolerance to heat, diarrhea , heart palpitations, and insomnia. Some patients experience protruding eyes and trembling. Treatments include medications to inhibit thyroid-hormone production, and removal or destruction of the thyroid gland with radioactive iodine. The most common cause of the excessive thyroid production is Graves' disease, an autoimmune disorder of the thyroid gland.
  • Addison's disease is caused by underactivity or immune-system destruction of the adrenal gland. It can be life-threatening if left untreated. Symptoms include weakness, fatigue, nausea, dehydration , fever , and hyperpigmentation (darkening of the skin without sun exposure.) The standard treatment is with corticosteroid hormones and adequate dietary salt.
  • Cushing's syndrome and Cushing's disease are different disorders with similar symptoms: obesity, weakness, easily bruised skin, acne, and hypertension (high blood pressure.) Cushing's syndrome is usually caused by excessive production of glucocorticoid hormones in the adrenal gland. However, it can sometimes be caused by benign or cancerous tumors of the adrenal gland. Cushing's disease usually results from the overproduction of the adrenocoticotropic hormone in the pituitary gland, due to a benign tumor. Treatment for both disorders can include surgery, radiation therapy, chemotherapy , and blocking production of the glucorticoid hormones with drugs.
  • Less common endocrine disorders include acromegaly, gigantism, and hypogonadism. Acromegaly occurs in adults and gigantism in children. Both are caused by a pituitary tumor that spurs overproduction of growth hormone. Hypogonadism causes delayed sexual maturity in children and infertility in adults. It is caused by underproduction of follicle-stimulating hormone (FSH) in the pituitary gland.

KEY TERMS


Adipose tissue —Connective tissue in which fat is stored and that has the cells distended by droplets of fat.

Autoimmune —A term that refers to a condition in which antibodies or T cells attack the molecules, cells, or tissue of the body organ or system producing them.

Electrolyte —A nonmetallic electric conductor in which current is carried by ion movement.

Lactation —The secretion of milk by the mammary gland in the breasts.

Lymphocytes —Weak cells produced in the lymphoid tissue.

Menopause —The period when natural menstruation stops, usually between ages of 45 and 50.

Menstruation —The discharge of blood from the uterus that occurs in approximately monthly intervals in females, starting at puberty.


Resources

BOOKS

Constanti, A., et al. Basic Endocrinology for Students of Pharmacy and Allied Health Sciences. Newark, NJ: Harwood Academic, 1998.

Hall, J. E., and L. Nieman, editors. Handbook of Diagnostic Endocrinology. Totowa, NJ: Humana Press, 2001.

Krentz, A. Churchill's Pocket Book of Diabetes. New York: Churchill Livingston, 2001.

Matthew, N. J. How the Endocrine System Works. Malden, MA: Blackwell Science, 2001.

Wilson, J. D., et al. Williams Textbook of Endocrinology. St. Louis, MO: W. B. Saunders, 1998.

PERIODICALS

American Diabetes Association. "Implications of the Diabetes Control and Complications Study." Diabetes Care (January 2001): 825.

Elliott, B. "Diagnosing and Treating Hypothyroidism." The Nurse Practitioner (March 2000): 92+.

Emanuele, N. and M. A. "The Endocrine System: Alcohol Alters Critical Hormonal Balance." Alcohol Health and Research World (Winter 1997): 53-64.

Hiller-Sturmhofel, S., and A. Bartke. "The Endocrine System: An Overview." Alcohol Health and Research World (Summer 1998): 153-154.

Lamberts, S. W. J., et al. "The Endocrinology of Aging." Science (October 17, 1997): 419-424.

Wilson, J. D. "Prospects for Research for Disorders of the Endocrine System." Journal of the American Medical Association (February 7, 2001): 624.

ORGANIZATIONS

American Association of Clinical Endocrinologists. 1000 Riverside Avenue, Suite 205, Jacksonville, FL 32204.(904) 353-7878. <http://www.aace.com>.

American Diabetes Association. 1701 North Beauregard Street, Alexandria, VA 22311. (800) 342-2383. <http://www.diabetes.org>.

American Thyroid Association. Montefiore Medical Center, 111 East 210th Street, Room 311, Bronx, NY 10467.(718) 882-6085. <http://www.thyroid.org>.

Endocrine Society. 4350 East West Highway, Suite 500, Bethesda, MD 20814. (301) 941-0200. <http://www.endo-society.org>.

Ken R. Wells

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Endocrine System." Gale Encyclopedia of Nursing and Allied Health. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"Endocrine System." Gale Encyclopedia of Nursing and Allied Health. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/endocrine-system-0

"Endocrine System." Gale Encyclopedia of Nursing and Allied Health. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/endocrine-system-0

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Endocrine System

Endocrine system

The endocrine system is the body's network of nine glands and over 100 hormones which maintain and regulate numerous events throughout the body. The glands of the endocrine system include the pituitary, thyroid, parathyroids, thymus, pancreas, pineal, adrenals , and ovaries or testes: in addition, the hypothalamus, in the brain , regulates the release of pituitary hormones. Each of these glands secrete hormones (chemical messengers) into the blood stream. Once hormones enter the blood, they travel throughout the body and are detected by receptors that recognize specific hormones. These receptors exist on target cells and organs. Once a target site is bound by a particular hormone, a cascade of cellular events follows that culminates in the physiological response to a particular hormone.

The endocrine system differs from the exocrine system in that exocrine glands contain ducts which direct their hormones to specific sites; whereas endocrine hormones travel through blood until they reach their destination. The endocrine is also similar to the nervous system , because both systems regulate body events and communicate through chemical messengers with target cells. However, the nervous system transmits neurotransmitters (also chemical messengers) between neighboring neurons via nerve extension, and neurotransmitters do not generally enter the circulation. Yet, some overlap between hormones and neurotransmitters exists which gives rise to chemical signals called neurohormones which function as part of the neuroendocrine system. The endocrine system oversees many critical life processes involving metabolism , growth, reproduction, immunity, and homeostasis . The branch of medicine that studies endocrine glands and the hormones which they secrete is called endocrinology.


History of endocrinology

Although some ancient cultures noted biological observations grounded in endocrine function, modern understanding of endocrine glands and how they secrete hormones has evolved only in the last 300 years. Ancient Egyptian and Chinese civilizations castrated (removed the testicles of) a servile class of men called eunuchs. It was noted that eunuchs were less aggressive than other men, but the link of this behavior to testosterone was not made until recently.

Light was shed on endocrine function during the seventeenth and eighteenth centuries by a few significant advances. A seventeenth century English scientist, Thomas Wharton (1614-1673), noted the distinction between ductile and ductless glands. In the 1690s, a Dutch scientist named Fredrik Ruysch (1638-1731) first stated that the thyroid secreted important substances into the blood stream. A few decades later, Theophile Bordeu (1722-1776) claimed that "emanations" were given off by some body parts that influenced functions of other body parts.

One of the greatest early experiments performed in endocrinology was published by A. A. Berthold (1803-1861) in 1849. Berthold took six young male chickens, and castrated four of them. The other two were left to develop normally and used comparatively as control samples. Two of the castrated chickens were left to become chicken eunuchs. But what Berthold did with the other two castrated chickens is what really changed endocrinology. He transplanted the testes back into these two chickens at a distant site from where they were originally. The two castrated chickens never matured into roosters with adult combs or feathers. But the chickens who received transplanted testes did mature into normal adult roosters. This experiment revealed that hormones that could access the blood stream from any site would function correctly in the body and that hormones did, in fact, travel freely in the circulation.

The same year Berthold published his findings, Thomas Addison (1793-1860), a British scientist reported one of the first well documented endocrine diseases which was later named Addison's disease (AD). AD patients all had a gray complexion with sickly skin; they also had weak hearts and insufficient blood levels of hemoglobin necessary for oxygen transport throughout the body. On autopsy, each of the patients Addison studied were found to have diseased adrenal glands. This disease can be controlled today if it is detected early. President John F. Kennedy suffered from AD.


Basic endocrine principles

Most endocrine hormones are maintained at specific concentrations in the plasma , the non-cellular, liquid portion of the blood. Receptors at set locations monitor plasma hormonal levels and inform the gland responsible for producing that hormone if levels are too high or too low for a particular time of day, month, or other life period. When excess hormone is present, a negative feedback loop is initiated such that further hormone production is inhibited. Most hormones have this type of regulatory control. However, a few hormones operate on a positive feedback cycle such that high levels of the particular hormone will activate release of another hormone. With this type of feedback loop, the end result is usually that the second hormone released will eventually decrease the initial hormone's secretion. An example of positive feedback regulation occurs in the female menstrual cycle , where high levels of estrogen stimulate release of the pituitary hormone, luteinizing hormone (LH).

All hormones are influenced by numerous factors. The hypothalamus can release inhibitory or stimulatory hormones the determine pituitary function. And every physiological component that enters the circulation can effect some endocrine function. Overall, this system uses multiple bits of chemical information to hormonally maintain a biochemically balanced organism .

Endocrine hormones do not fall into any one chemical class, but most are either a protein (polypeptides, peptides, and glycoproteins are also included in this category) or steroids. Protein hormones bind cell-surface receptors and activate intracellular events that carry out the hormone's response. Steroid hormones, on the other hand, usually travel directly into the cell and bind a receptor in the cell's cytoplasm or nucleus. From there, steroid hormones (bound to their receptors) interact directly with genes in the DNA to elicit a hormonal response.



The pituitary

The pituitary gland has long been called "the master gland," because it secretes multiple hormones which, in turn, trigger the release of other hormones from other endocrine sites. The pituitary is roughly situated behind the nose and is anatomically separated into two distinct lobes, the anterior pituitary (AP) and the posterior pituitary (PP). The entire pituitary hangs by a thin piece of tissue , called the pituitary stalk, beneath the hypothalamus in the brain. The AP and PP are sometimes called the adenohypophysis and neurohypophysis, respectively.

The PP secretes two hormones, oxytocin and antidiuretic hormone (ADH), under direction from the hypothalamus. Direct innervation of the PP occurs from regions of the hypothalamus called the supraoptic and paraventricular nuclei. Although the PP secretes its hormones into the bloodstream through blood vessels that supply it, it is regulated in a neuroendocrine fashion. The AP, on the other hand, receives hormonal signals from the blood supply within the pituitary stalk.

AP cells are categorized according to the hormones that they secrete. The hormone-producing cells of the AP include: somatotrophs, corticotrophs, thyrotrophs, lactotrophs, and gonadotrophs. Somatotrophs secrete growth hormone; corticotrophs secrete adrenocorticotropic hormone (ACTH); thyrotrophs secrete thyroid stimulating hormone (TSH); lactotrophs secrete prolactin; and gonadotrophs secrete LH and follicle stimulatory hormone (FSH). Each of these hormones sequentially signals a response at a target site. While ACTH, TSH, LH, and FSH primarily stimulate other major endocrine glands, growth hormone and prolactin primarily coordinate an endocrine response directly on bones and mammary tissue, respectively.

The pineal

The pineal gland is a small cone-shaped gland believed to function as a body clock. The pineal is located deep in the brain just below the rear-most portion of the corpus callosum (a thick stretch of nerves that connects the two sides of the brain). The pineal gland, also called the pineal body, has mystified scientists for centuries. The seventeenth century philosopher, Rene Descartes, speculated that the pineal was the seat of the soul. However, its real function is somewhat less grandiose than that.

The pineal secretes the hormone melatonin, which fluctuates on a daily basis with levels highest at night. Although its role is not well understood, some scientists believe that melatonin helps to regulate other diurnal events, because melatonin fluctuates in a 24-hour period. Exactly what controls melatonin levels is not well understood either; however, visual registration of light may regulate the cycle.


The thyroid

The thyroid is a butterfly-shaped gland that wraps around the back of the esophagus. The two lobes of the thyroid are connected by a band of tissue called the isthmus . An external covering of connective tissue separates each lobe into another 20-40 follicles. Between the follicles are numerous blood and lymph vessels in another connective tissue called stroma. The epithelial cells around the edge of the follicles produce the major thyroid hormones.

The major hormones produced by the thyroid are triiodothyronine (T3), thyroxine (T4), and calcitonin. T3 and T4 are iodine-rich molecules that fuel metabolism. The thyroid hormones play several important roles in growth, metabolism, and development. The thyroid of pregnant women often become enlarged in late pregnancy to accommodate metabolic requirements of both the woman and the fetus.

Thyroid hormones accelerate metabolism in several ways. They promote normal growth of bones and increase growth hormone output. They increase the rate of lipid synthesis and mobilization. They increase cardiac output by increasing rate and strength of heart contractions. They can increase respiration , the number of red blood cells in the circulation, and the amount of oxygen carried in the blood. In addition, they promote normal nervous system development including nerve branching.


The parathyroids

While most people have four small parathyroid glands poised around the thyroid gland, about 14% of the population have one or two additional parathyroid glands. Because these oval glands are so small, the extra space occupied by extra glands does not seem to be a problem. The sole function of these glands is to regulate calcium levels in the body. Although this may seem like a simple task, the maintenance of specific calcium levels is critical. Calcium has numerous important bodily functions. Calcium makes up 2-3% of adult weight with roughly 99% of the calcium in bones. Calcium also plays a pivotal role in muscle contraction and neurotransmitter secretion.


The thymus

In young children, the thymus extends into the neck and the chest, but after puberty , it begins to shrink. The size of the thymus in most adults is very small. Like some other endocrine glands, the thymus has two lobes connected by a stalk. The thymus secretes several hormones that promote the maturation of different cells of the immune system in young children. In addition, the thymus oversees the development and "education" of a particular type of immune system cell called a T lymphocyte, or T cell.

Although many details of thymal hormonal activity are not clear, at least four thymal products have been identified which control T cell and B cell (antibody-producing immune cells) maturation. The four products are: thymosin, thymic humoral factor (THF), thymic factor (TF), and thymopoietin. Because the viral disease, AIDS , is characterized by T cell depletion, some AIDS treatment approaches have tried administering tymosin to boost T cell production.


The pancreas

The pancreas is a large endocrine and exocrine gland situated below and behind the stomach in the lower abdomen. The pancreas is horizontally placed such that its larger end falls to the right and its narrower end to the left. Clusters of exocrine pancreatic cells called acini secrete digestive enzymes into the stomach; while endocrine cells secrete hormones responsible for maintaining blood glucose levels.

The endocrine cells of the pancreas are contained in the islets of Langerhans which are themselves embedded in a rich network of blood and lymph vessels. About one million islet cells make up about 2% of the total pancreas: the islet cells are quite small. The three major types of endocrine cells within the islets are alpha cells, beta cells, and delta cells. Beta cells make up about 70% of islet cells and secrete insulin . Alpha cells which secrete glucagon comprise roughly 20% of islet cells. Delta cells, which comprise up to 8% of islet cells, secrete somatostatin. Another pancreatic hormone, called pancreatic polypeptide, is secreted by F cells in the islets and has just recently been characterized.

Insulin is secreted in response to high plasma glucose levels. Insulin facilitates glucose uptake into blood cells thus reducing plasma glucose levels. Glucagon has the opposite effect; low plasma glucose triggers the breakdown of stored glucogen in the liver and glucose release into the blood. By balancing these two hormones, the islets continually regulate circulating glucose levels. Both hormones are contained within secretory vesicles in the cells which release them. And monitoring of blood glucose concentrations are evaluated directly at the pancreas, as opposed to being mediated by another gland such as the pituitary. In addition, nerve endings at the islets contribute in regulating insulin and glucagon secretion. The hormone somatotostatin is also released under conditions of increased blood glucose, glucagon, and amino acids. Somatostatin inhibits additional insulin release.

The overall effect of insulin is to store fuel in muscle and adipose tissue. It also promotes protein synthesis. Hence, insulin is said to be anabolic which means that it works to build up instead of break down energy-storing molecules. Glucagon, however, is metabolic, since it promotes glycogen breakdown. However, each of the pancreatic hormones is also considered to be paracrine, since they also modulate other pancreatic cells.

Diabetes mellitus (DM) is a very serious endocrine disorder caused by insulin dysfunctions. In type I DM, the beta cells are defective and can not produce enough insulin. Type II DM is caused by a lack of target cell-receptor responsiveness to available insulin. While type I requires regular insulin injections, type II can be controlled with diet.


The adrenals

One of the two adrenals sit atop each kidney and are divided into two distinct regions, the cortex and the medulla. The outer area makes up about 80% of each adrenal and is called the cortex. And the inner portion is called the medulla. The adrenals provide the body with important buffers against stress while helping it adapt to stressful situations.

The cells of the cortex form three distinct layers, the zona glomerulosa (ZG), the zona fasciculata (ZF), and the zona reticularis (ZR). All three layers secrete steroid hormones. The ZG secretes mineralocorticoids; the ZF secretes glucocorticoids; and the ZR secretes primary androgens. Cholesterol droplets are interspersed throughout the cortex for synthesis of cortical steroids. ACTH released from the anterior pituitary triggers glucocorticoids release from the ZF. The major glucocorticoid in humans is cortisol, which fluctuates in concentration throughout the day: the highest levels exist in the morning around 8 a.m. with the lowest levels around midnight. Both physical and emotional stress can affect cortisol secretion. The major human mileralocorticoid is aldosterone. Aldosterone acts on the kidneys and is important in regulating fluid balance. It triggers increased extracellular fluid that leads to increased blood pressure .

Cells of the adrenal medulla, called chromaffin cells, secrete the hormones epinephrine (adrenaline) and non-epinephrine (nor-adrenaline). Chromaffin cells are neuroendocrine cells which function like some nerve fibers of the sympathetic nervous system. However, these cells are endocrine, because the neurohormones that they release target distant organs. Although the effects of these two medullary hormones are the same whether they originate in the endocrine or the nervous system, endocrine hormonal effects are prolonged, because they are removed more slowly from blood than from a nerve terminal. Both cortical and medullary hormones work together in emergencies, or stressful situations, to meet the physical demands of the moment.


The ovaries

The ovaries are located at the end of each fallopian tube in the female reproductive tract, and they produce the female reproductive hormones: estrogen, progesterone, and relaxin. Although the fluctuation of these hormones is critical to the female menstrual cycle, they are initially triggered by a hormone from the hypothalamus, called a releasing factor, that enables gonadotrophs in the pituitary to release LH and FSH that, in turn, regulate part of the menstrual cycle. All of these hormones work together as part of the endocrine system to ensure fertility. They are also important for the development of sexual characteristics during puberty.

Each month after puberty, females release a single egg (ovulation) when the pituitary releases LH. Prior to ovulation, the maturing egg releases increasing levels of estrogen that inform the pituitary to secrete LH. While an egg travels down the fallopian tube, progesterone is released which prevents another egg from beginning to mature. Once an egg is shed in the uterine lining (in menstruation), the cycle can begin again. During pregnancy, high levels of circulating estrogen and progesterone prevent another egg from maturing. Estrogen levels fall dramatically at menopause , signifying the end of menstrual cycling and fertility. Menopause usually occurs between the ages of 40 and 50.

Endocrine regulation of the female reproductive tract does not stop during pregnancy. In fact, more sex hormones are released during pregnancy than during any other phase of female life. At this time, a new set of conditions support and protect the developing baby. Even cells of the developing embryo begin to release some hormones that keep the uterine lining intact for the first couple of months of pregnancy. High progesterone levels prevent the uterus from contracting so that the embryo is not disturbed. Progesterone also helps to prepare breasts for lactation. Towards the end of pregnancy, high estrogen levels stimulate the pituitary to release the hormone, oxytocin, which triggers uterine contractions. Prior to delivery, the ovaries release the hormone, relaxin, a protein which causes the pelvic ligaments to become loose for labor.


The testes

The two testes are located in the scrotum, which hangs between the legs behind the penis. Most of the testes is devoted to sperm production, but the remaining cells, called Leydig cells, produce testosterone. Testosterone caries out two very important endocrine tasks in males: it facilitates sexual maturation, and it enables sperm to mature to a reproductively-competent form. Healthy men remain capable of fertilizing an egg throughout their post-pubertal life. However, testosterone levels do show a gradual decline after about the age of 40 with a total drop of around 20% by age 80.

Testosterone also has the important endocrine function of providing sexual desire in both men and women. Although reproduction can occur without maximal desire, this added incentive increases the likelihood of reproduction. Human sexual behavior is also influenced by several other factors including thoughts and beliefs.


Endocrine disorders

As much as 10% of the population will experience some endocrine disorder in their lifetime. Most endocrine disorders are caused by a heightened or diminished level of particular hormones. For example, excess growth hormone can cause giantism (unusually large stature). Tumors in endocrine glands are one of the major causes of hormone overproduction.

Hormone underproduction is often due to a mutation in the gene that codes for a particular hormone or the hormone's receptor; even if the hormone is normal, a defective receptor can render the hormone ineffective in eliciting a response. The distinction between the pancreatic endocrine disorders, diabetes mellitus types I and II, make this point very clearly. In addition, underproduction of growth hormones can lead to dwarfism. Insufficient calcitonin from the thyroid can also lead to cretinism which is also characterized by diminished stature due to low calcium availability for bone growth.

The importance of diet can not be overlooked in some endocrine disorders. For example, insufficient dietary iodine (required for T3 and T4 synthesis) can cause goiters. Goiters are enlarged thyroid glands caused by the thyroid's attempt to compensate for low iodine with increased size. Certain endocrine imbalances can also cause mental problems ranging from poor ability to concentrate to mental retardation (as in cretinism).

See also Chemoreception.

Resources

books

Little, M. The Endocrine System. New York: Chelsea House Publishers, 1990.

Marieb, Elaine Nicpon. Human Anatomy & Physiology. 5th Edition. San Francisco: Benjamin/Cummings, 2000.


Louise Dickerson

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exocrine

—Glands with ducts that direct hormones to a specific target.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Endocrine System." The Gale Encyclopedia of Science. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"Endocrine System." The Gale Encyclopedia of Science. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/endocrine-system-0

"Endocrine System." The Gale Encyclopedia of Science. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/endocrine-system-0

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Endocrine System

Endocrine System


The endocrine system is made up of a number of glands that control bodily functions by secreting hormones. It is found in all vertebrates (animals with a backbone) and works together with the nervous system to regulate many of the body's functions. The hormones secreted by glands are chemicals that carry instructions from one set of cells in an animal's body to another. The endocrine system can be described as the body's chemical coordinating system.

Although the endocrine system works closely with the body's nervous system in helping it respond appropriately to a changing environment, it is different from that system in many ways. Where nerve transmission is rapid and over quickly, the effect generated by a certain hormone can last several days and is usually concerned with large, long-term processes or situations. For example, the nervous system makes a child jerk his hand away from a hot stove almost as soon as he or she has touched it. In a very different situation however, it is the endocrine system that senses that the child has not been fed for some time and makes sure that his or her brain has the steady supply of glucose (sugar) that he or she needs. Therefore, the messages sent by the endocrine system usually have long-term effects and are almost always concerned with the body's larger processes, unlike the nervous system which generally brings about short-term changes.

THE ENDOCRINE SYSTEM AND HORMONES

In vertebrates, the endocrine system uses several different glands, as well as specialized tissue and cells, to secrete hormones directly into the bloodstream. These glands are called ductless because they have no need for ducts or tubelike connections to the circulatory system (a network that carries blood throughout the body). Rather, they secrete hormones in very small amounts that travel throughout the body via the bloodstream to eventually reach their "target organ" (which is sometimes not close to the secreting gland). Although the hormones cause no changes in the tissue they pass through, they do cause major changes in the target organs, which have special receptors or receivers for hormones. Working together with the nervous system, the hormones released by the body's endocrine system regulate growth, ovulation (the release of an ovum or egg from an ovary), milk production, sexual development, and many other processes. The endocrine system can be considered the body's chemical coordinating system as it gives orders to start and stop certain bodily changes as needed. Since stopping a process is sometimes as important as starting, the endocrine system has what are called feedback mechanisms that monitor a situation and signal when a gland should stop secreting hormones. Malfunctions in the system can cause an over or undersupply of a hormone, resulting in such conditions as gigantism (unusually large stature), dwarfism (unusually small stature), or a goiter (visible swelling at the front of the neck).

Hormones also play a large part in the growth cycle of insects. Since insects have an exoskeleton (a hard shell that surrounds its body), they must periodically molt or shed their skeleton if they are to grow. Molting involves the precise coordination of many separate steps, and hormones begin and coordinate the entire, complicated process. A hormone also signals caterpillars to make a cocoon and enter their pupa (resting) stage. It is in this stage that they undergo a metamorphosis and transform into an entirely different creature, like a moth or butterfly.

GLANDS OF THE ENDOCRINE SYSTEM

In humans and other vertebrates, the endocrine system regulates a great deal more than one or two bodily functions and processes. In humans, there are at least nine separate glands that make up the endocrine system, and they are scattered throughout the body. These major endocrine glands are: the pituitary, the hypothalamus, the pineal, the thyroid, the parathyroid, the thymus, the adrenal, the pancreas, and the gonads.

The Pituitary. Located at the base of the brain (of which it is actually a part), the pituitary influences so many glands that it has been called the "master gland." About the size of a pea, this gland works with the hypothalamus as part of a direct link between the endocrine system and the nervous system. Although small, it is really two glands in one and its parts are called the anterior (front) and the posterior (rear) pituitary. The anterior secretes the hormone prolactin, which makes the female body ready to produce milk. The anterior also secretes five other hormones that start other glands working. The posterior pituitary secretes oxytocin, which makes the uterus contract during birth and stimulates the release of milk. Vasopressin which regulates the balance of water and raises blood pressure is also released by the posterior pituitary.

The Hypothalamus. The hypothalamus monitors internal organs and emotional states and supervises the release of hormones from the anterior pituitary gland. It also produces substances called releasing factors that control hormonal secretions from other glands.

The Pineal. The pineal gland is a light-sensitive organ that evolved from a third eye that vertebrates had on top of their heads until about

240,000,000 years ago. It secretes the hormone melatonin, which acts as the body's biological clock and helps regulate its rhythms.

The Thyroid. The thyroid is located around the windpipe at the front of the neck and produces hormones containing iodine that are necessary for growth, development, and proper metabolism (all the body's processes involved in using energy). Hypothyroidism, or a lack of these hormones, leads to stunted growth and retardation in children. In adults, hypothyroidism produces dry skin, sluggish behavior, and an inability to tolerate cold. Hyperthyroidism, or an overproduction of hormones, results in nervousness and agitation, weight loss, and heavy sweating.

The Parathyroid. There are four parathyroid glands next to the thyroid. They regulate the level of calcium in the blood by taking calcium stored in bones and releasing it into the bloodstream. Proper calcium levels are essential to muscle contraction and the clotting of blood.

The Thymus. The thymus is located behind the breastbone and between the lungs. It is often considered to be part of the immune system since it plays a major role in the development of lymphocytes (white blood cells) that fight infection. It is also considered part of the endocrine system because it produces a hormone that stimulates the growth of these white cells.

The Adrenals. The two adrenal glands found in humans are located above each kidney, and their outer region, called the cortex, secretes hormones called glucocorticoids. These help maintain the blood's important glucose level and also minimize inflammation (swelling and tenderness) that is caused by infection or injury. The adrenal medulla is the inner region of the gland and secretes epinephrine and norepinephrine, which prepare the body for stress by narrowing the blood vessels and increasing both the heart rate and the amount of glucose in the blood. During times of major excitement, these dramatic changes can be easily felt and recognized as the body prepares for "fight or flight."

The Pancreas. The pancreas (an organ involved in digestion) is located below the stomach and contains clusters of endocrine cells called pancreatic islets that secrete glucagon and insulin. These hormones are key to controlling the level of glucose in the bloodstream. When not enough insulin is produced or the target cells do not respond to insulin, a disease called diabetes mellitus is diagnosed.

The Gonads. Finally, the gonads are the main organs of reproduction and secrete sex hormones. The testes in males and the ovaries in females usually come in pairs in most mammals. The testes are responsible for producing androgens—the most important of which is testosterone—which help develop sperm and are responsible for what are called secondary sexual characteristics, like a deeper voice and facial hair. The ovaries help in the production of ova (eggs cells) and produce progesterone and estrogen. These hormones are responsible for female development (like breasts) and also control pregnancy. Both of these physical changes come about in boys and girls during puberty, that burst of development that occurs during the early teenage years.

THE IMPORTANCE OF THE ENDOCRINE SYSTEM

It is thought that vertebrates have as many as fifty different hormones, suggesting that the endocrine system of humans is a highly complex and very intricate system. It might be said that the overall goal or purpose of the endocrine system is to maintain a balance in all the body's systems. Despite this general goal, the hormones that the system actually releases can have powerful and even dramatic effects. For example, the gonads change a boy into a man and a girl into a woman; the adrenal gland pumps adrenalin and can make a person unnaturally strong or not feel pain; the pituitary can produce nutritious breast milk to nourish a baby; and the thymus can help wage and win a war against infection.

[See alsoHormone; Immune System; Reproductive System; Sex Hormones ]

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Endocrine System." U*X*L Complete Life Science Resource. . Encyclopedia.com. 16 Nov. 2018 <https://www.encyclopedia.com>.

"Endocrine System." U*X*L Complete Life Science Resource. . Encyclopedia.com. (November 16, 2018). https://www.encyclopedia.com/social-sciences/applied-and-social-sciences-magazines/endocrine-system

"Endocrine System." U*X*L Complete Life Science Resource. . Retrieved November 16, 2018 from Encyclopedia.com: https://www.encyclopedia.com/social-sciences/applied-and-social-sciences-magazines/endocrine-system

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.