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Endocrine glands are glands of the endocrine system that secrete their products, ''hormones'', directly into the blood rather than through a duct. The major glands of the endocrine system include the pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus and adrenal glands. The hypothalamus and pituitary gland are neuroendocrine organs. Local chemical messengers, not generally considered part of the endocrine system, include autocrines, which act on the cells that secrete them, and paracrines, which act on a different cell type nearby. The ability of a target cell to respond to a hormone depends on the presence of receptors, within the cell or on its plasma membrane, to which the hormone can bind. Hormone receptors are dynamic structures. Changes in number and sensitivity of hormone receptors may occur in response to high or low levels of stimulating hormones. Blood levels of hormones reflect a balance between secretion and degradation/excretion. The liver and kidneys are the major organs that degrade hormones; breakdown products are excreted in urine and feces. Hormone half-life and duration of activity are limited and vary from hormone to hormone. ==Interaction of hormones at target cells== ''Permissiveness'' is the situation in which a hormone cannot exert its full effects without the presence of another hormone. ''Synergyism'' occurs when two or more hormones produce the same effects in a target cell and their results are amplified. ''Antagonism'' occurs when a hormone opposes or reverses the effect of another hormone. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Endocrine gland」の詳細全文を読む スポンサード リンク
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