Bryan Cowles, age 14, of Greenville, Miss., for her question:

WHAT DOES A SPIROMETER DO?

A spirometer is a device that doctors use to measure how much oxygen a person's body uses. It also measures how much air the lungs can hold, how well the lungs are working and the number of times a person breathes each minute.

There are two cylinders on a spirometer. One cylinder is upside down and rests inside a larger cylinder that is filled with water. The patient breathes through a rubber tube into the inner cylinder. The added air causes the cylinder to rise in the water.

When the patient inhales., he removes air from the cylinder and it sinks. A writing arm records these movements on a piece of paper. This record is called a sprirogram.

The spirometer is used most often to measure a person's basal metabolic rate, which is the amount of oxygen the body uses while at rest.

To measure how much air the lungs can hold, the patient first breathes in as deeply as possible and then exhales as much air as possible into the spirometer. The amount of air that was exhaled is called the vital capacity. The vital capacity is less than normal if a person has such disorders as asthma or certain heart diseases.