The function of endocrine systems is to maintain homeostasis of the organism and to perpetuate the species. Homeostasis is maintained by the continual adjustment of nervous and endocrine systems in response to a changing environment. Because most endocrine systems are intimately related to the central nervous system and the autonomic nervous system (see Chap. 8,Chap. 9 and Chap. 85), a continual bidirectional flow of information—one relaying the conditions in the organism, and the other directing neural and endocrine responses—is present and functions to maintain appropriate operating conditions.