The endocrine system broadcasts its hormonal messages to essentially all cells by secretion into blood and extracellular fluid. Like a radio broadcast, it requires a receiver to get the message - in the case of endocrine messages, cells must bear a receptor for the hormone being broadcast in order to respond. As will be repeatedly demonstrated, the nervous and endocrine systems often act together to regulate physiology. Indeed, some neurons function as endocrine cells. Endocrinology is the study of hormones, their receptors and the intracellular signalling pathways they invoke. Distinct endocrine organs are scattered throughout the body. These are organs that are largely or at least famously devoted to secretion of hormones, and no introduction to endocrinology would be complete without some kind of endocrine organ "map" such as that below: