We have seen how the body receives information about its internal and external environments through its various sensory systems. We also have examined how the central nervous system processes all the incoming information it receives and then sends out appropriate response commands via its autonomic nervous system (affecting smooth muscles and organs) or its voluntary nervous system (affecting skeletal muscles). This chapter completes the circuit by addressing how longer-term orders are given by the central nervous system and carried out by hormones produced by endocrine glands. The net result of all this activity is that the central nervous system regulates essentially all body activities and unites a diverse collection of cells, tissues, and organs into a smoothly functioning, unified, single organism. The functioning of each organ system is controlled by feedback loops and antagonistic mechanisms, thus maintaining the homeostasis necessary for life. Many hormones are chemical extensions of the nervous system and bring about long-term changes or responses within the organism's body. Not all hormones, however, are under the control of the nervous system, and in fact the nervous system itself is influenced by hormones.