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 controlled by hormones 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.
Objective Checklist

The Endocrine System and Hormone Function: An Overview

• Define hormone and target organ.
• Describe how hormones bring about their effects in the body.
• Explain how various endocrine glands are stimulated to release their hormonal products.
• Define negative feedback and describe its role in regulating blood levels of the various hormones.

The Major Endocrine Organs

• Describe the difference between endocrine and exocrine glands.
• On an appropriate diagram, identify the major endocrine glands and tissues.
• List hormones produced by the endocrine glands and discuss their general functions.
• Discuss ways in which hormones promote body homeostasis by giving examples of hormonal actions.
• Describe the functional relationship between the hypothalamus and the pituitary gland.
• Describe major pathological consequences of hypersecretion and hyposecretion of the hormones considered in this chapter.

Other Hormone-Producing Tissues and Organs

• Indicate the endocrine role of the kidneys, the stomach and intestine, the heart, and the placenta.

Developmental Aspects of the Endocrine System

• Describe the effect of aging on the endocrine system and body homeostasis.