Hormones
Hormones are organic substances that are secreted by plants and animals and that function in the regulation of physiological activities and in maintaining homeostasis. They carry out their functions by evoking responses from specific organs or tissues that are adapted to react to minute quantities of them. The classical view of hormones is that they are transmitted to their targets in the bloodstream after discharge from the glands that secrete them. This mode of discharge (directly into the bloodstream) is called endocrine secretion. The meaning of the term hormone has been extended beyond the original definition of a blood-borne secretion, however, to include similar regulatory substances that are distributed by diffusion across cell membranes instead of by a blood system. . Among animals, the hormones of the vertebrates--particularly those of humans and other mammals--are the best known. Most vertebrate hormones originate in specialized tissues, called endocrine tissues, and are carried to their targets through the bloodstream. Endocrine glands. A major endocrine gland in vertebrates is the pituitary, which consists of two distinct sections: the anterior pituitary (or adenohypophysis) and the po
Because many proteins function as enzymes within the cell, this influence on protein synthesis can have far-reaching effects on the cell's activities. This second messenger--in many cases, cyclic adenosine monophosphate (cyclic AMP)--apparently activates enzyme systems that bring about the desired action by the cell. Its designation as the master gland notwithstanding, the anterior pituitary itself is regulated by substances called releasing hormones that are secreted by the hypothalamus, the part of the brain located directly above the pituitary. These hypothalamic hormones stimulate--or, in some cases, inhibit--the secretions of the anterior pituitary. It is interesting to note that both proposed mechanisms of hormone action involve the binding of the hormone to a specific receptor molecule. This feature accounts for the specificity of hormones; a hormone can have an effect only on cells that possess the appropriate receptor. Together with FSH and LH, these hormones control the cyclical changes in the female reproductive system--the menstrual cycle in human females and the estrous cycle in other female mammals. This hormone stimulates glands in the thorax to secrete the hormone ecdysone, which causes the periodic molting, or shedding, of the hard exoskeleton. (see also Index: thyroid hormone, parathormone) The female sex hormones--the estrogens and progesterone--are produced by the ovaries. The posterior pituitary stores and releases two hormones: oxytocin, which causes the uterus to contract during birth, and vasopressin, which acts on the kidneys to restrict the output of urine. Progesterone is concerned with the maintenance of pregnancy. sterior pituitary (or neurohypophysis). These two hormones are actually produced by the hypothalamus, which is linked directly to the posterior pituitary. Hormones from the adrenal cortex regulate glucose and sodium metabolism. Other endocrine glands in vertebrates include the thyroid, parathyroids, adrenals, pancreas, and gonads (sex glands).
Common topics in this essay:
Hormones Hormones,
FSH LH,
anterior pituitary,
hormones hormones,
endocrine glands,
posterior pituitary,
hormone-receptor complex,
vertebrate hormones,
enter target cell,
secretes hormones affect,
master gland,
hormones affect,
receptor protein,
targets bloodstream,
|
Acceptance Essays
