The nature of the endocrine glands, the interaction of the nervous and endocrine systems, and the actions of hormones will be discussed in detail in later chapters. For now, it is sufficient to describe the regulation of hormone secretion very broadly, since it so superbly illustrates the principles of homeostasis and negative feedback regulation.
Hormones are secreted in response to specific chemical stimuli. A rise in the plasma glucose concentration, for example, stimulates insulin secretion from structures in the pancreas known as the pancreatic islets, or islets of Langerhans. Hormones are also secreted in response to nerve stimulation and to stimulation by other hormones.
The secretion of a hormone can be inhibited by its own effects, in a negative feedback manner. Insulin, as previously described, produces a lowering of blood glucose. Since a rise in blood glucose stimulates insulin secretion, a lowering of blood glucose caused by insulin's action inhibits further insulin secretion. This closed-loop control system is called negative feedback inhibition (fig. 1.6a).
Homeostasis of blood glucose is too important—the brain uses blood glucose as its primary source of energy—to entrust to the regulation of only one hormone, insulin. So, during fasting, when blood glucose falls, it is prevented from falling too far by several mechanisms (fig. 1.6b). First, insulin secretion decreases, preventing muscle, liver, and adipose cells from taking too much glucose from the blood. Second, the secretion of a hormone antagonistic to insulin, called glucagon, increases. Glucagon stimulates processes in the liver (breakdown of a stored, starchlike molecule called glycogen—see chapter 2) that cause it to secrete glucose into the blood. Through these and other antagonistic negative feedback mechanisms, the blood glucose is maintained within a homeostatic range.
Test Yourself Before You Continue
1. Define homeostasis and describe how this concept can be used to explain physiological control mechanisms.
2. Define the term negative feedback and explain how it contributes to homeostasis. Illustrate this concept by drawing a negative feedback loop.
3. Describe positive feedback and explain how this process functions in the body.
4. Explain how the secretion of a hormone is controlled by negative feedback inhibition. Use the control of insulin secretion as an example.
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