John Bruining, age 17, of Ottawa, Ontario, Canada, for his question:
WHAT CAUSES THIRST?
A sensation called thirst is caused by your body's need for water. Your internal environment, as the organs and tissues inside your body are called, needs certain amounts of water and salt to function properly. And thirst makes sure the water is delivered.
Too much or too little water, or too much or too little salt, can damage or even kill body cells. The sensation of thirst provides an automatic way for animals and people to satisfy the need and control the intake of these elements.
You experience thirst by having a dry feeling in your mouth and throat. A lack of enough saliva can also produce this kind of thirst even when your internal environment has no immediate need for water.
The thirst sensation caused by the internal environment's need will disappear for a short period of time if water is drunk and it wets the mouth and throat. But unless the water reaches the internal environment, the thirst sensation will come back after a short period of time.
Sensory nerves in your internal organs are affected by the volume of fluid in your internal environment. These nerves help regulate the amount of water that you drink.
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 very important in maintaining the internal environment. It holds nerve cells that respond to changes in the amount of salt in the blood. The salt concentration of the body may indicate how much water is in the cells of the body.
A decrease in body water will produce an increase in the salt concentration in the blood. When this happens, the amount of water in the internal environment is affected and less water is then eliminated from the body.
When you are thirsty, you naturally have a great desire to drink. Under some conditions, however, water may be harmful. After strenuous exercise, for example, the drinking of too much water can cause the remaining salt in the body to further diminish. Water intoxication, or cramps and weakness, may result. In such cases, salt tablets and citrus fruit drinks might be taken.
Sometimes thirst is caused by eating salty foods. Other times it can be caused by breathing through the mouth. And an altered chemical composition of the blood may also sometimes induce thirst when there is no real need for water.
Diabetics get thirsty because fluid is lost in the urine in order to keep the excreted sugar in solution.
After a severe internal or external hemorrhage, the patient almost always has a great thirst. And the same thing happens when there is traumatic shock.