ETYM From Greek ormaein to excite.
Secretion of the endocrine glands, concerned with control of body functions. The major glands are the thyroid, parathyroid, pituitary, adrenal, pancreas, ovary, and testis. Hormones bring about changes in the functions of various organs according to the body’s requirements. The hypothalamus, which adjoins the pituitary gland, at the base of the brain, is a control center for overall coordination of hormone secretion; the thyroid hormones determine the rate of general body chemistry; the adrenal hormones prepare the organism during stress for “fight or flight”; and the sexual hormones such as estrogen govern reproductive functions.
There are also hormone secreting cells in the kidney, liver, gastrointestinal tract, thymus (in the neck), pineal (in the brain), and placenta. Many diseases due to hormone deficiency can be relieved with hormone preparations.
Secretin.
The first hormone to be discovered was secretin, a substance released by the small intestine when food reaches it from the stomach. Two English physiologists, Sir William Bayliss and Ernest Starling, showed in 1905 that the secretin passed into the circulation, reached (after going round the whole body) the pancreas, and stimulated it to produce juice, which enabled the food to be broken down and absorbed in the intestine. In this way, digestive juice is only produced at appropriate times, when food is present. Bayliss and Starling showed that blood taken from an animal which was digesting a meal (whose blood therefore contained secretin), when injected into the circulation of an animal which had not eaten recently, made the second animal's pancreas secrete juice.
Discovery and absence of hormones.
Since 1905 many more hormones have been discovered; some, like insulin and aldosterone, are essential for life. Many more are not; their absence is only a source of varying degrees of inconvenience, and it is difficult to find a function at all for some hormones, e.g. oxytocin in males. (Oxytocin's only known roles are in females—to produce contraction of the uterus in childbirth, and to help the release of milk from the breast during lactation.).
Interaction.
The relationships between hormones can be very complex, for some endocrine glands produce hormones that stimulate other glands to produce hormones; this is how the anterior pituitary gland controls the thyroid gland, the adrenal cortex, the testis, and the ovary. When stimulated by the relevant pituitary hormone (known as a trophic hormone), each one of these glands produces its own hormones which have their own effects. But these hormones do something else that is highly ingenious; they act on the pituitary gland itself. They actually decrease (inhibit) the pituitary’s secretion of the trophic hormone. In this way the amount of circulating hormone is kept constant. This is known as “negative feedback”, a feature of many control systems in biology and engineering. Negative feedback is used by the body to keep many of its constituents at a constant level; it is exactly analogous to the thermostat system used for keeping the temperature of a house within narrow limits. Another illustration occurs in the disease.
Diabetes insipidus.
Disease has played an important role in our understanding of hormones, for historically the first evidence of a hormone's existence has often been an unexplained disease which subsequently turns out to be caused by over- or under-secretion of the hormone. The secretion of hormones has been implicated in the production of some forms of cancer, particularly of the breast and uterus in women, and of the prostate gland in men.
An endocrine secretion that is transmitted by the blood to the tissue on which it has a specific effect; SYN. internal secretion.