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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 ]

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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.

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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.

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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.

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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.

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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 )

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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.

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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.

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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.

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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.

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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).

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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.

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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.

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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.

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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.

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endocrinology

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

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endocrinology

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

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endocrine gland

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

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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

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