Jane Ferguson, age 10, of Chester, Penna., for her question:

HOW DO WE BREATHE?

Breathing, or respiration as it is called, is the taking in of oxygen and the giving off of carbon dioxide. In human beings and many animals, breathing continues constantly without any conscious effort. It is a rhythmic, automatic procedure.

Tiny cells at the base of the brain control the breathing process. Like a robot working a pump, these operate the muscles of the chest as it expands and contracts.

Air is warmed and moistened as it is drawn into the body by the nose. You can also breathe through your mouth. To keep food from entering the air passage, the epiglottis acts as a safety door. It closes when food is swallowed.

The air is drawn into the body though a tube called the trachea. This tube passes through the neck and air is taken to the lungs by way of a series of bronchial tubes.

The air from the bronchial tubes flows into millions of air sacs, called alveoli, which make up the spongelike mass of each lung. Each sac is covered with a network of tiny blood vessels called capillaries. These blood vassals take the oxygen from the air into the blood stream.

Air enters the lungs when the diaphragm, a large muscle attached to the lower ribs that separates the chest from the abdomen, moves down and the chest muscles lift the ribs. Air is squeezed out again when the diaphragm lifts and the ribs drop.

Thanks to our breathing pattern, blood entering the lungs carries a waste gas called carbon dioxide while blood leaving the lungs contains a fresh supply of oxygen.

All living cells need oxygen in order to carry out their various functions.

The breathing process is called external respiration. The exchange of gases is called internal respiration or tissue respiration. A third form of respiration, called cellular respiration, occurs within the cells of the body.

Breathing is actually made up of two separate acts: inspiration, which is breathing in, and expiration, which is breathing out. The chest muscles act to expand and contract the chest cavity, causing the lungs to fill or empty. A pause occurs between inspiration and expiration. The pause is shorter during fast breathing.

For a person to breathe in, his chest must expand. The lungs fill the chest cavity completely, no matter how large it is. So when the chest expands, the lungs do also.

This expansion lowers the air pressure inside the lungs, creating a slight vacuum that pulls the air from the atmosphere. To do this, the diaphragm contracts, pulling its dome downward, providing more room in the cheat cavity.

At the same time, the muscles surrounding the ribs contract. The ribs move upward and outward, making the chest cavity expand frontward.

Expiration results when the process of inspiration is reversed.

A person breathing quietly inhales and exhales about one pint of air at each breath.