How Hormones Work
Hormones act as chemical messengers and are secreted into the bloodstream by the endocrine glands for distribution around the body. Each hormone acts only upon specific types of cells---those bearing the correct membrane receptor sites. These are the "target cells" for the hormone. Within the bloodstream there are many different hormones. When specific receptor sites are not filled, cells remain "turned off". As hormones attach to receptor sites, they initiate a "switch on" reaction in the cell cytoplasm---this reaction is usually the production of the chemical cyclic adenosine mnophosphate or cyclic AMP from ATP. Cyclic AMP acts as a messenger--within the cell-- to activate the cell's production lines. The activity in the bloodstream of some hormones, such as adrenaline, may last for only ten minutes. Others, including some sex hormones, have a longer life. Hormones may be inactivated by the target cells or by the liver, and the breakdown products, when their actions have been completed, are either excreted or reused in the further manufacture of hormones.