Endocrinology is the study of chemical communication systems that provide the means to control a huge number of physiologic processes. Like other communication networks, endocrine systems contain transmitters, signals and receivers that are called, respectively hormone producing cells, hormones and receptors.
             What exactly are hormones and how are they different from "non-hormones"? Hormones are chemical messengers secreted into blood or extracellular fluid by one cell that affect the functioning of other cells.
             Most hormones circulate in blood, coming into contact with essentially all cells. However, a given hormone usually affects only a limited number of cells, which are called target cells. A target cell responds to a hormone because it bears receptors for the hormone.
             In other words, a particular cell is a target cell for a hormone if it contains functional receptors for that hormone, and cells which do not have such a receptor cannot be influenced directly by that hormone. Reception of a radio broadcast provides a good analogy. Everyone within range of a transmitter for National Public Radio is exposed to that signal (even if they don't contribute!). However, in order to be a NPR target and thus influenced directly by their broadcasts, you have to have a receiver tuned to that frequency.
             Hormone receptors are found either exposed on the surface of the cell or within the cell, depending on the type of hormone. In very basic terms, binding of hormone to receptor triggers a cascade of reactions within the cell that affects function. Additional details about receptor structure and function are provided in the section on hormone mechanism of action.
             · Endocrine action: the hormone is distributed in blood and binds to distant target cells.
             · Paracrine action: the hormone acts locally by diffusing from its source to target cells in the neighborhood.
             · Autocrine action: the hormone acts on the same ...