394 CHAPTER 9• Endocrine and Nervous Systems
ANATOMY AND PHYSIOLOGY OVERVIEW
The endocrine and nervous systems work together like interlocking supersystems to control many intricate activities
of the body. Together they monitor changes in the body and in the external environment, interpret these changes,
and coordinate appropriate responses to reestablish and maintain a relative equilibrium in the internal environment
of the body (homeostasis).
The endocrine system consists of a network of glandular structures that slowly discharge hormones into the blood-
stream. In contrast, the nervous system is designed to act instantaneously by transmitting electrical impulses to specific
body locations, which helps control all critical body activities and reactions.
ENDOCRINE SYSTEM
The endocrine system is a network of ductless glands that have a rich blood supply, which enables the hormones they
produce to enter the bloodstream. Although hormone production occurs at one site, the effects of these hormones
take place at various other sites in the body. Processes under direct endocrine control include growth, reproduction,
cellular metabolism, and the regulation of blood levels of many important nutrients. The tissues or organs that respond
to the effects of a hormone are called target tissues,or target organs. (See Fig. 9–1.)