Alan Sawyer, age 8, of Baltimore, Md., for his question:

HOW MANY TIMES A MINUTE DOES THE HEART BEAT?

If body cells didn't receive the right amount of oxygen, they would stop working. Sometimes the body needs lots of oxygen while at other times it needs little. The heart responds automatically to these needs of the body.

An adult man's heart normally beats about 70 times each minute. But the rate changes to provide as much oxygen as his body needs.

The body needs a lot of fuel for such strenuous exercises as running or playing tennis. For this reason, the heart beats quickly when a person runs. The heart is rushing more oxygen to the body by speeding the blood's flow.

If the person stops exercising, his heart gradually slows down. It is regulating the flow of blood to the body's slower tempo.

In infants, the heart beats about 120 times per minute and in young children, the rate is about 90 per minute.

The rate of beat of the heart changes to meet a particular need. The heart beats faster when a person is angry, afraid or excited. It rushes more oxygen to the person's muscles, preparing him for fighting, running or some other action.