The regulation of the coordinated effort between the various organs, cells, and systems within the human body is a remarkable communications structure. The messengers that transport the signals for all manner of activity between the various centers of the body are the chemicals known as hormones. Hormones are also known by their biological classification, endocrines, which are defined as the chemicals that are carried by either blood or by the tissue structures of the body to a determined place, to initiate a predetermined action. These structures are in contrast to the exocrines, which include the saliva and the tear duct products of the body; the exocrine structures do not direct their product within the body, but expel chemicals from the body.

Hormones are produced by the small organs known as glands. Glands produce the chemicals used to signal desired changes in cellular or system performance through the receipt of materials from the bloodstream and then fashioning the particular hormone specific to the gland. The important glandular structures of the body are organized into a hierarchy. The impetus for most hormonal production begins with the signals that are transmitted by the hypothalamus, the portion of the brain that is responsible for the coordination of many forms of organ and system responses.

When the hypothalamus determines that the body must take action of some kind in response to an external stimulus, such as an athlete preparing to compete in a soccer game, a series of chemical signals are transmitted from the hypothalamus to the organ known as the "master gland," the pituitary. The pituitary is responsible for both the ongoing maintenance and regulation of several critical functions, including the production of the human growth hormone (HGH). The pituitary also regulates the function of the glands that ultimately provide the distinct chemical responses directed by the hypo-thalamus, the thyroid gland and the adrenal glands.

Insulin and glucagon are hormones produced in the pancreas, the small organ located below the stomach. These chemicals are substances that operate in contrast; insulin is the regulatory agent essential to the maintenance of an optimal glucose balance, signaling the liver to store more glucose when the blood sugar level is too high, whereas glucagon conveys the opposite signal, to release greater amounts of glucose into the bloodstream, when the levels are too low. In an ideal state, the influences of insulin and glucagon upon blood glucose levels are balanced.

The female ovaries and the male testes also have glands that are related to human reproduction. Each of these organs contain endocrine cells that permit the organ to produce the hormones necessary to produce eggs in the case of the female, and sperm cells in the case of the male.

The hormones produced by the glands are one of two general types: protein hormones (which include related substances known as peptides and modified amino acids), and steroid hormones, composed of various types of ringed, carbon-based molecules. The distinct structure of each determines how these hormones are physically transported through the bloodstream and tissues of the body. While insulin and glucagon operate in reference to the present level of one another (antagonistically), many important hormones are secreted in response to the presence or absence of the substance to which they are directed.

The male sex hormone, testosterone, contributes to the maintenance of the body's general energy levels necessary for effective function in all physical activities.

Other important hormones and their glandular production sites include the pituitary, pineal, thyroid, and adrenal glands, as well as erythropoietin (EPO). The pituitary gland (located in the brain) functions as the master gland, with a distinct regulatory function in relation to the thyroid and adrenal glands, as well as the production of the human growth hormone, the substance that regulates both the rate and timing of growth patterns within the body. The pineal gland produces melatonin, a hormone that is important in the regulation of the production of other hormones. The thyroid gland (found in the neck) regulates a variety of related metabolic and energy-generating functions, through the production of a number of specific hormones. The adrenal glands (one located at each kidney) produce adrenaline, the "fight or flight response" hormone, which influences heart rate, respiration, and other organ functions when adrenaline is released, in response to circumstances of excitement or threat; the adrenal glands also produce the hormonal direction to the kidneys to conserve water and sodium and to maintain the balance between those substances in the body when the glands are signaled that there may be imminent dehydration. Erythropoietin (EPO) is produced in the kidneys. This hormone will trigger increased production of erythrocytes (red blood cells), whose increased numbers within the bloodstream provide the body with a greater capacity to transport oxygen, to counter the effects of high altitudes, and to correspondingly reduce oxygen levels upon the aerobic and anaerobic energy systems.

Hormonal imbalances sometimes occur within the body. Such imbalances may be caused by external factors, such as improper diet, or through natural processes such as female menopause.

SEE ALSO Cardiovascular system; High altitude effects on sport performance; Hormonal response to exercise.