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The Merck Manual of Medical Information--Home Edition
Section 13. Hormonal Disorders
Chapter 143

Endocrine System and Hormones

The endocrine system consists of a group of organs (sometimes referred to as glands of internal secretion) whose main function is to produce and secrete hormones directly into the bloodstream. Hormones serve as messengers to coordinate activities of various parts of the body.

Endocrine Glands

click on thumbnail to see full-size imageThe major organs of the endocrine system are the hypothalamus, the pituitary gland, the thyroid gland, the parathyroid glands, the islets of the pancreas, the adrenal glands, the testes, and the ovaries. During pregnancy, the placenta also acts as an endocrine gland in addition to its other functions.

The hypothalamus secretes several hormones that stimulate the pituitary: Some trigger the release of pituitary hormones; others suppress the release of pituitary hormones.

The pituitary gland is sometimes called the master gland because it coordinates many functions of the other endocrine glands. (see illustration, page 697) Some pituitary hormones have direct effects; others simply control the rate at which other endocrine organs secrete their hormones. The pituitary controls the rate at which it secretes its own hormones through a feedback loop in which the blood levels of other endocrine hormones signal the pituitary to slow down or speed up. Not all endocrine glands are under the pituitary's control; some respond directly or indirectly to concentrations of substances in the blood:

  • The insulin-secreting cells of the pancreas respond to glucose and fatty acids.
  • Parathyroid cells respond to calcium and phosphate.
  • The adrenal medulla (part of the adrenal gland) responds to direct stimulation by the parasympathetic nervous system.

Many organs secrete hormones or hormonelike substances but aren't generally considered part of the endocrine system. Some of these organs produce substances that act only in the immediate vicinity of their release, while others don't secrete their products into the bloodstream. For example, the brain produces many hormones whose effects are confined mainly to the nervous system.


The Function of Messengers

Although all cells respond to and most cells produce messengers, their effects are usually grouped into three major systems--the nervous, immune, and endocrine systems--essential for coordinating the body's activities. These three systems have much in common and cooperate closely. Their messengers are made up of proteins or derivatives of fat. Some messengers travel only a fraction of an inch; others travel a considerable distance through the bloodstream to reach their target.

The messengers attach themselves to their target cells using specific receptor proteins located either on the cell surface or within the cell. Some messengers alter the permeability of cell membranes to specific substances; for example, insulin alters the transport of glucose across cell membranes. Other messengers, such as epinephrine and glucagon, modify the activity of their receptors, causing them to produce other substances that act as second messengers. They affect the activity of the cell's genetic material, altering the cell's production of proteins or the activity of proteins already in the cell.

The effect of a given messenger depends on where it's secreted. For example, norepinephrine raises the blood pressure when the adrenal gland secretes it into the blood, but when norepinephrine is released within the nervous system, it stimulates the activity of only nearby nerve cells without affecting the blood pressure.

Hormones are substances released into the bloodstream from a gland or organ that affect activity in cells at another site. Most hormones are proteins composed of amino acid chains of varying length. Others are steroids, fatty substances derived from cholesterol. Very small amounts of hormones can trigger very large responses in the body.

Hormones bind to receptors on a cell's surface or inside a cell. The binding of a hormone to a receptor speeds up, slows down, or in some other way alters the cell's function. Ultimately, hormones control the function of entire organs. They control growth and development, reproduction, and sexual characteristics. They influence the way the body uses and stores energy. Hormones also control the volume of fluid and the levels of salt and sugar in the blood. Some hormones affect only one or two organs, while others affect the whole body. For example, thyroid-stimulating hormone is produced in the pituitary gland and affects only the thyroid gland. In contrast, thyroid hormone is produced in the thyroid gland, but it affects cells throughout the body. Insulin, produced by the islet cells of the pancreas, affects the metabolism of glucose, protein, and fat throughout the body. (see box Major Hormones in this chapter)

Endocrine Controls

When endocrine glands malfunction, hormone levels in the blood can become abnormally high or low, disrupting body functions. To control endocrine functions, the secretion of each hormone must be regulated within precise limits. The body needs to sense from moment to moment whether more or less of a given hormone is needed.

The hypothalamus and pituitary glands secrete their hormones when they sense that the blood level of another hormone that they control is too high or too low. Pituitary hormones then travel through the bloodstream to stimulate activity in their target glands. When the target hormone's level in the blood is appropriate, the hypothalamus and pituitary gland recognize that no further stimulation is needed and they stop secreting hormones. This feedback system regulates all glands under pituitary control.

Certain hormones under pituitary control vary according to schedules. For example, a woman's menstrual cycle involves monthly fluctuations in the pituitary gland's secretion of luteinizing hormone and follicle-stimulating hormone. The ovarian hormones estrogen and progesterone also fluctuate monthly. Exactly how the hypothalamus and pituitary control these biorhythms isn't understood. However, the organs clearly respond to some sort of biologic clock.

Other factors also stimulate hormone production. Prolactin, a hormone secreted by the pituitary gland, causes milk glands in the breast to produce milk. A baby sucking on the nipple stimulates the pituitary gland to secrete more prolactin. The sucking also increases the secretion of oxytocin, which causes the milk ducts to contract, moving milk to the nipple for the hungry baby.

Glands such as the islets of the pancreas and the parathyroid glands, which aren't under pituitary control, have their own systems for sensing whether more or less hormone secretion is needed. For example, insulin levels increase shortly after eating because the body needs to process the sugars from the food. However, if insulin levels were to remain high, then blood sugar levels would fall dangerously low.

Other hormone levels vary for less obvious reasons. Corticosteroid and growth hormone levels are highest in the morning and lowest in midafternoon. Reasons for these daily variations aren't fully understood.

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