Hormones

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A hormone is a chemical that is produced in one tissue and transported via the circulatory system to a different target tissue. There, it causes a physiological change in the target.

Hormones are the chemical messengers of the endocrine system. The endocrine system also includes the ductless glands that synthesize and secrete hormones, and incorporates the responding target cells as well. Hormones are secreted by endocrine glands directly into the circulatory system, from which they contact nearly all cells of the body. Some endocrine glands, such as the adrenal glands, form organs of their own, while others are just parts of organs. The brain, for example, performs certain critical endocrine functions.

The endocrine system is one of two physiological systems responsible for the control of all biological processes. The other is the nervous system. While the nervous system controls specific, rapid biological responses, often to external stimuli, endocrine control generally involves comparatively broad, long-term, gradual physiological processes.

The endocrine system is essential to diverse aspects of an organism's biology, including its development, growth, reproduction, metabolism, water and ionic balance, and maintenance of homeostasis (internal equilibrium). In general, animal species that are characterized by well-developed nervous and circulatory systems also possess endocrine control systems.

Because hormones are transported through the circulatory system, they come into contact with all cells and are able to affect numerous tissues simultaneously. Some hormones affect a wide variety of tissues. The sex hormone testosterone, for example, affects multiple parts of the body, whereas others have a considerably more limited effect.

Only cells that possess receptors specific to a hormone will respond to its presence. In addition, depending on the hormone receptor and the pathway coupled to it, different tissues can respond to the same hormone in different ways. Thus, despite their relatively low concentrations in the bloodstream, hormones can have dramatic effects on an organism's physiology.

Hormones have been divided into two major groups that differ in their biochemical attributes, as well as in the mechanisms by which they affect the activity of target cells. These are steroid hormones and peptide hormones.

Steroid hormones are synthesized by endocrine glands in the gonads (ovaries and testes) and adrenal cortex. They are not stored but, rather, secreted into the circulatory system as soon as they are synthesized.

Steroid hormones are derived from cholesterol and are lipid soluble. Lipid solubility enables steroid hormones to cross cell membranes and enter directly into the cytoplasm. Once there, hormone molecules bind to cytoplasmic receptors, cross the nuclear membrane, and interact directly with DNA to affect cellular activity. Some well-known steroids are estrogen and testosterone.

Peptide hormones, on the other hand, are proteins and composed of amino acids. Peptide hormones are water soluble and range greatly in size. They are synthesized in endocrine cells and then stored in vesicles within the cell for secretion later.

Peptides are the more diverse group of hormones by far. Unlike steroids, peptide hormones are not lipid soluble and do not penetrate their target cells directly. Instead, they function via what is referred to as a second messenger pathway. The hormone binds to a receptor protein on the target cell membrane, which then signals a second messenger within the cellular cytoplasm. This second messenger initiates an enzyme cascade, which affectsthe activity of the cell. Examples of second messengers involved in peptide hormone function include cyclic AMP and inositol triphosphate.

The maintenance of appropriate hormone concentrations in the bloodstream is absolutely critical. Numerous diseases result from hormone levels that are too high or too low. Diabetes is one well-known example.

Feedback systems are often used to regulate hormone synthesis and secretion. Some of these cycles can be extremely complex, involving numerous hormones and endocrine glands.

A particularly well-studied example is the control of thyroid hormone levels. The hypothalamus, an endocrine organ in the brain, secretes a hormone called the thyroid-releasing hormone (TRH). TRH targets the anterior pituitary, which responds by secreting thyroid-stimulating hormone (TSH).

TSH targets the thyroid, inducing it to secrete the thyroid hormones known as T3 and T4. However, when T3 and T4 reach a certain concentration in the bloodstream, they act on the hypothalamus, inhibiting it from secreting more TRH. As a result, TSH is no longer secreted, and T3 and T4 secretion is also terminated. This type of negative feedback is common in endocrine regulation. When the levels of thyroid hormones fall below a certain concentration in the bloodstream, the inhibitory, or restraining, effect on the hypothalamus is removed.

The hypothalamus and the anterior pituitary (which is often referred to as the master gland) are critical to endocrine control because many of the hormones they produce affect the activity of other endocrine glands. The hypothalamus is located at the base of the middle portion of the brain, and the pituitary lies immediately below it. The two are directly connected by blood vessels, an unusual organization of the circulatory system referred to as a portal system. The portal system allows for the direct and efficient transport of hormones from the hypothalamus to targets within the pituitary.

Other hormones are under cyclical control. Cycles can be short, lasting hours, or much longer, spanning several months. Melatonin is a hormone produced by the pineal gland whose level follows a daily cycle. It establishes circadian rhythms. Hormone cycling over longer periods is responsible for the control of activities such as menstruation, hibernation, and seasonal mating behavior.

One major endocrine gland is the anterior pituitary. It secretes growth hormone as well as gonadotropins, which stimulate sex hormone production in the gonads, and prolactin, which is associated with lactation. Another important endocrine gland is the posterior pituitary. It secretes antidiuretic hormone, one of the key players involved in water balance, and oxytocin, which induces uterine contractions during childbirth.

Other significant endocrine glands can be cited. The thyroid is responsible for the thyroid hormones T3 and T4, which regulate growth, development, and metabolism. Of the adrenal glands, the adrenal medullaproduces epinephrine and norepinephrine, while the adrenal cortex produces steroid hormones including the mineralocorticoids and glucocorticoids. The pancreas secretes insulin and glucagon, two antagonistic hormones that together regulate blood glucose levels. Finally, there are the thymus, the pineal gland, and the ovaries and testes, which produce sex hormones.

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