In a living organism there must be coordination of a number of physiological activities taking place simultaneously, such as: Central nervous system and endocrine system represent the two major means by which these functions are coordinated. Communication b/w cells that are not in contact is achieved through a number of chemical substances. Interact w/ specific receptors on distant target cells. Signaling through these receptors leads to a specific physiological effect: endocrine signaling: Involves hormone secretion into blood by endocrine gland. Hormone is transported by blood to distant target site. Communication by hormones (or neurohormones) can involve six steps. Step 4 and 5 is the essence of endocrinology. All sites of potential regulation of endocrine signaling (1) synthesis of hormone by endocrine cells. Or neurons in case of neurohormone (2) release of hormone by endocrine cells. Anp: hormone that controls (in part) sodium balance and blood pressure.