▸ A Living Example Could biological systems achieve homeostasis through feedback inhibition? Absolutely. All that is needed is a system that regulates some aspect of the cellular environment and that can respond to feedback from its own activities by switching on or off as needed. Such mechanisms are very common, not only in the human body, but in all forms of life.

One example is the maintenance of body temperature. The body regulates temperature by a mechanism that is remarkably similar to that of a home heating system. You can follow body temperature regulation in Figure 30–4. A part of the brain called the hypothalamus contains nerve cells that monitor both the temperature of the skin at the surface of the body and the temperature of organs in the body's core.

If the nerve cells sense that the core temperature has dropped much below 37°C, the hypothalamus produces chemicals that signal cells throughout the body to speed up their activities. Heat produced by this increase in activity, especially cellular respiration, causes a gradual rise in body temperature, which is detected by nerve cells in the hypothalamus.

Have you ever been so cold that you began to shiver? If your body temperature drops well below its normal range, the hypothalamus releases chemicals that signal muscles just below the surface of the skin to contract involuntarily—to “shiver.” These muscle contractions release heat, which helps the body temperature to rise toward the normal range.

If body temperature rises too far above 37°C, the hypothalamus slows down cellular activities to reduce heat production. This is one of the reasons you may feel tired and sluggish on a hot day. The body also responds to high temperatures by producing sweat, which helps to cool the body surface by evaporation.

FIGURE 30–4 Body Temperature Control In the human body, temperature is controlled through various feedback inhibition mechanisms. Infer Why do you think moving around on a cold day helps to keep you warm?

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