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 ]