Christine Cook, age 15, of Greenville, Miss., for her question:

HOW DOES HUMAN RESPIRATION WORK?

Respiration is the physical process by which living organisms take in oxygen from the surrounding medium and emit carbon dioxide. In humans, lungs take in air. The principal nervous center for controlling the rate and depth of respiration in humans is in the respiratory section of the vague nucleus in the medulla oblongata of the brain. Human respiration is an automatic procedure.

The lunge are enclosed in the thorax. The ribs support the body wall of the thorax, which has a domed base formed by the diaphragm. The ribs slant downward and forward. When they're raised by the action of the intercostal muscles, the volume of the thorax is increased.

The volume of the thorax is also increased by contraction of the muscles of the diaphragm. Within the thorax, the lungs are held close to the body wall by atmospheric pressure and, when the thorax expands, the lungs also expand and become filled with air drawn through the upper respiratory passages.

Relaxation of the muscles expanding the thorax allows the opposing set of muscles to return the chest to its natural contracted position, forcing the air from the lungs.

From 12 to 30 cubic inches of sir are usually inhaled and exhaled at each inspiration. This volume is called tidal sir. About 90 cubic inches of additional air, called complemental air, can be inhaled on a forced inspiration and then exhaled. Still another 90 cubic inches of additional air can be exhaled on a forced expiration.

The sum of these three quantities is called the vital capacity. About 90 cubic inches of air always remains in the lungs and cannot be exhaled. This volume is called residual air.

The human lungs are roughly pyramidal in shape, conforming to the shape of the thorax. They are not strictly symmetrical. The right lung consists of three lobes while the left has two lobes and, near the medial edge of the base, a cardiac notch into which the heart extends.

On the medial side of each lung is the root, by which the lung is attached to the mediastinum, or central partition of the chest. The root consists of pleural folds, bronchi and pulmonary arteries and veins.

As the bronchus penetrates the substance of the lung, it divides and subdivides repeatedly until it ends 1n the lobule, the structural anti functional unit of the lung.

Accompanying the bronchus, the pulmonary arteries and veins divide at the same points, the aterioles and venules of the lobules being connected by a dense network of capillaries that lie in the walls of the air cells.

Nerves from the pulmonary plexus and lymphatic vessels are also distributed in the same manner.

Within the lobule, the bronchiole divides into terminal bronchi, each of which opens into a group of atria, or air spaces. Each of the atria opens in turn into a number of alveolar sacules, the walls of which are punched out to form the numerous aveole, or air cells, of the lobule.