hormone

hormone secretory substance carried from one gland or organ of the body via the bloodstream to more or less specific tissues, where it exerts some influence upon the metabolism of the target tissue. Normally, various hormones are produced and secreted by the endocrine glands (see endocrine system ), including the pituitary, thyroid, parathyroids, adrenals, ovaries, testes, pancreatic islets, certain portions of the gastrointestinal tract, and the placenta, among the mammalian species. As lack of any one of them may cause serious disorders, many hormones are now produced synthetically and used in treatment where a deficiency exists. The hormones of the anterior pituitary include thyrotropin , adrenocorticotropic hormone , the gonadotropic hormones , and growth hormone ; the posterior pituitary secretes antidiuretic hormone , prolactin, and oxytocin . The thyroids secrete thyroxine and calcitonin, and the parathyroids secrete parathyroid hormone . The adrenal medulla secretes epinephrine and norepinephrine while the cortex of the same gland releases aldosterone , corticosterone , cortisol , and cortisone . The ovaries primarily secrete estrogen and progesterone and the testes testosterone . The adrenal cortex, ovaries, and testes in fact produce at least small amounts of all of the steroid hormones. The islets of Langerhans in the pancreas secrete insulin , glucagon , and somatostatin. The kidneys also produce erythropoietin, which produces erythrocytes (red blood cells). The passage of chyme (see digestive system ) from the stomach to the duodenum causes the latter to release secretin, which stimulates the flow of pancreatic juice. The duodenum can also be stimulated by the presence of fats in the chyme to secrete cholecystokinin, a hormone that stimulates the gall bladder to contract and release bile . There is evidence that the upper intestine secretes pancreatozymin, which enhances the amount of digestive enzymes in the pancreatic juice. In addition, the pyloric region of the stomach secretes gastrin, a hormone that increases the secretion of hydrochloric acid into the stomach. The placenta has been shown to secrete progesterone and chorionic gonadotropin. There is evidence that it even contains a substance similar to growth hormone. Insects have a unique hormonal system that includes ecdysone, a steroid that influences molting and metamorphosis, and juvenile hormone, needed for early development. Plants, too, have a hormonal system, which includes the auxins , the gibberellins, the cytokinins, and substances associated with the formation of flowers, tubers, bulbs, and buds. Ethylene is said to function as a hormone in plants, acting to hasten the ripening of fruits.

hormone Chemical substance secreted by living cells.Hormones affect the metabolic activities of cells in other parts of the body. In mammals, glands of the endocrine system secrete hormones directly into the bloodstream. Hormones exercise chemical control of physiological functions, regulating growth, development, sexual functioning, metabolism, and (in part) emotional balance. They maintain a delicate equilibrium that is vital to health. The hypothalamus, adjacent to the pituitary gland at the base of the brain, is responsible for overall coordination of the secretion of hormones. Most hormones are proteins or steroids. Hormones include thyroxine, adrenaline, insulin, oestrogen, progesterone, and testosterone. In plants, hormones control many aspects of metabolism, including cell elongation and division, direction of growth, initiation of flowering, development of fruits, leaf fall, and responses to environmental factors. The most important plant hormones include auxin, gibberellin, and cytokinin. See also homeostasis

hormone
1. A substance that is manufactured and secreted in very small quantities into the bloodstream by an endocrine gland or a specialized nerve cell (see neurohormone) and regulates the growth or functioning of a specific tissue or organ in a distant part of the body. For example, the hormone insulin controls the rate and manner in which glucose is used by the body. Other hormones include the sex hormones, corticosteroids, adrenaline, thyroxine, and growth hormone.

2. A plant growth substance.