9.1. Role of Hormones in Regulation of Metabolism and Functions
The classic definition of hormones of endocrine glands is that they are organic molecules that are synthesized in specialized glands and secreted directly in the bloodstream to act on target tissues. They function as chemical mediators to relay signals about changes in the external and internal environment to various organs and tissues. The cell response to the hormone action is very diverse and is determined by both the chemical structure of the hormone and the type of cell that has the hormone action is directed to. Hormones circulate in the plasma in very low concentrations (picomolar to micromolar range). Therefore, to exert physiological effects of hormones on target tissues, the cells must contain receptors on their plasma membranes or within their interior, which recognize and bind the circulating hormones with high affinity and specificity. The «life» of a hormone and duration of its physiological effect is dependant on various factors, for example, the hormone concentration (that is determined by the synthesis and inactivation rate, and the rate of excretion of hormones and their metabolites from the body), its affinity for transport proteins (steroid and thyroid hormones are carried along bloodstream combined with proteins), the number and type of receptors on the target cells surface.
Hormones are synthesized in response to biochemical signals generated by various systems of an organism. For example, insulin synthesis is regulated by the glucose