Chapter 9: The Endocrine System

You don't have to watch CSI to experience action-packed drama. Your body cells have dynamic adventures on microscopic levels all the time. For instance, when insulin molecules, carried passively along in the blood, leave the blood, and bind tightly to protein receptors of nearby cells, the response is dramatic: bloodborne glucose molecules begin to disappear into the cells, and cellular activity accelerates. Such is the power of the second great controlling system of the body, the endocrine system. Along with the nervous system, it coordinates and directs the activity of the body's cells. However, the speed of control in these two great regulating systems is very different. The nervous system is "built for speed." It uses nerve impulses to prod the muscles and glands into immediate action so that rapid adjustments can be made in response to changes occurring both inside and outside the body. On the other hand, the more slowly acting endocrine system uses chemical messengers called hormones, which are released into the blood to be transported leisurely throughout the body.

Although hormones have widespread and varied effects, the major processes they control are reproduction; growth and development; mobilizing body defenses against stressors; maintaining electrolyte, water, and nutrient balance of the blood; and regulating cellular metabolism and energy balance. As you can see, the endocrine system regulates processes that go on for relatively long periods and, in some cases, continuously. The scientific study of hormones and endocrine organs is called endocrinology.