Gretchen Moore, age 13, of Wilmington, Del., for her question:

WHY DO WE FEEL THRIST AT TIMES?

Thirst is a sensation caused by the body's need for water.

People often describe thirst as a dry feeling in the throat. A lack of sufficient saliva can produce this kind of thirst, even when the internal environment's need will disappear briefly if water is drunk and wets the throat. But unless the water reaches the internal environment, thirst will recur after a short time.

The sensory nerves in the internal organs are affected by the volume of fluid in the internal environment. These nerves help regulate the amount of water drunk.

The internal senses tell how much water and salt are inside and outside the cells of the body. One source of this information is a region of the brain stem called the hypothalamus. The hypothalamus is important in maintaining the internal environment. It contains nerve cells that respond to changes in the amount of salt in the blood.

The salt concentration of the blood may indicate how much water is in the cells of the body. For example, a decrease in body water produces an increase in the salt concentration in the blood. Thus, the amount of water in the internal environment also affects the amount of water eliminated from the body.