If the body is to maintain homeostasis and function effectively, its trillions of cells must work together in a coordinated fashion. Should each cell behave without regard to what others were doing, the result would be physiological chaos and death. Two communication systems prevent this–the nervous and endocrine systems. A particularly important aspect of these systems is that they can detect changes in an organ system and modify its physiology as well as the physiology of other systems. Thus, they functionally unite the systems of the body and play a central coordinating role in homeostasis. The nervous system is the focus of this chapter and the three that follow; the endocrine system is covered in chapter 17.