The study of the mechanical, physical, and biochemical functions of the Endocrine System is called Endocrine Physiology. Endocrine disorders involve the body’s over- or under-production of certain hormones, while metabolic disorders affect the body’s ability to process certain nutrients and vitamins. The function of the endocrine system is to coordinate and integrate cellular activity within the whole body by regulating cellular and organ function throughout life and maintaining homeostasis. Homeostasis, or the maintenance of a constant internal environment, is critical to ensuring appropriate cellular function. The endocrine system broadcasts its hormonal messages to essentially all cells by secretion into blood and extracelular fluid. Most homones circulate in the blood, coming into contact with essentially all cells. However, a given hormone usually effects only a limited number of cells, which are called target cells. A target cell responds to a hormone because it bears receptors for that hormone. Endocrine system is that of an integrated network of multiple organs derived from different embryologic origins that release hormones ranging from small peptides to glycoproteins, which exert their effects either in neighboring or distant target cells. This endocrine network of organs and mediators does not work in isolation and is closely integrated with the central and peripheral nervous systems as well as with the immune systems.