- A chemical substance secreted by an endocrine gland or group of endocrine cells that acts to control or regulate specific physiological processes, including growth, metabolism, and reproduction. Most hormones are secreted by endocrine cells in one part of the body and then transported by the blood to their target site of action in another part, though some hormones act only in the region in which they are secreted. Many of the principal hormones of vertebrates, such as growth hormone and thyrotropin, are secreted by the pituitary gland, which is in turn regulated by neurohormone secretions of the hypothalamus. Hormones also include the endorphins, androgens, and estrogens.
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- A substance that is synthesized by a plant part and acts to control or regulate the growth and development of the plant. The action and effectiveness of a hormone can depend on the hormone's chemical structure, its amount in relation to other hormones that have competing or opposing effects, and the ways in which it interacts with chemical receptors in various plant parts. Auxins, cytokinins, gibberellins, abscisic acid, and ethylene are plant hormones.
A Closer Look Among the most abundant and influential chemicals in the human body are the hormones, found also throughout the entire animal and plant kingdoms. The endocrine glands alone, including the thyroid, pancreas, adrenals, ovaries, and testes, release more than 20 hormones that travel through the bloodstream before arriving at their targeted sites. The pea-sized pituitary gland, located at the base of the brain below the hypothalamus, is considered the most crucial part of the endocrine system, producing growth hormone and hormones that control other endocrine glands. Specialized cells of the nervous system also produce hormones. The brain itself releases endorphins, hormones that act as natural painkillers. Hormones impact almost every cell and organ of the human body, regulating mood, growth, tissue function, metabolism, and sexual and reproductive function. Compared to the nervous system, the endocrine system regulates slower processes such as metabolism and cell growth, while the nervous system controls more immediate functions, such as breathing and movement. The action of hormones is a delicate balancing act, which can be affected by stress, infection, or changes in fluids and minerals in the blood. The pituitary hormones are influenced by a variety of factors, including emotions and fluctuations in light and temperature. When hormone levels become abnormal, disease can result, such as diabetes from insufficient insulin or osteoporosis in women from decreased estrogen. On the other hand, excessive levels of growth hormone may cause uncontrolled development. Treatment for hormonal disorders usually involves glandular surgery or substitution by synthetic hormones.