Needless to say, this doesn't happen, because the homeostatic mechanism controlled by the hypothalamus intervenes again. When the water content of the blood rises, the pituitary releases less ADH. In response to lower ADH levels, the kidneys remove water from the blood, restoring the blood to its proper concentration. This homeostatic system sets both upper and lower limits for blood water content. A water deficit stimulates the release of ADH, causing the kidneys to conserve water; an oversupply of water causes the kidneys to eliminate the excess water in urine.

Controlling Metabolism As another example of how internal feedback mechanisms regulate the activity of the endocrine system, let's look at the thyroid gland and its principal hormone, thyroxine. Recall that thyroxine increases the metabolic activity of cells. Does the thyroid gland determine how much thyroxine to release on its own? No, the activity of the thyroid gland is instead controlled by the hypothalamus and the anterior pituitary gland. When the hypothalamus senses that the thyroxine level in the blood is low, it secretes thyrotropin-releasing hormone (TRH), a hormone that stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH). TSH stimulates the release of thyroxine by the thyroid gland. High levels of thyroxine in the blood inhibit the secretion of TRH and TSH, which stops the release of additional thyroxine. This feedback loop keeps the level of thyroxine in the blood relatively constant.

The hypothalamus is also sensitive to temperature. When the core body temperature begins to drop, even if the level of thyroxine is normal, the hypothalamus produces extra TRH. The release of TRH stimulates the release of TSH, which stimulates the release of additional thyroxine. Thyroxine increases oxygen consumption and cellular metabolism. The increase in metabolic activity that results helps the body maintain its core temperature even when the outside temperature drops.

34.2 Assessment

Review Key Concepts

1. 1. Review Describe the role of each major endocrine gland.

   2. Explain How is the hypothalamus an important part of both the nervous system and the endocrine system?

   3. Compare and Contrast Compare and contrast the two types of diabetes.

2. 1. Review Explain how the endocrine system helps maintain homeostasis.

   2. Explain On a hot day, you play soccer for an hour and lose a lot of water in sweat. List the steps that your body takes to regain homeostasis.

   3. Predict Suppose the secretion of a certain hormone causes an increase in the concentration of substance X in the blood. A low concentration of X causes the hormone to be released. What is the effect on the rate of hormone secretion if an abnormal condition causes the level of X in the blood to remain very low?

WRITE ABOUT SCIENCE

Creative Writing

3. Create a brochure that describes both types of diabetes. You may wish to include information on risk factors, treatment, and preventive measures that can be taken. Use images from magazines or the Internet to illustrate your brochure.

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