You breathe to supply your body with oxygen and to get rid of carbon dioxide, a waste product of energy produc­tion. The lungs are the center of the respiratory system. In your lungs, the oxygen in the air you breathe is exchanged for carbon dioxide from the blood. The channel that the air follows in and out of your lungs is called the respiratory tract. It includes the nose, throat, and trachea, or wind­pipe. Deep in the chest the trachea divides into two main tubes, which are called bronchi. Each one goes into one lung, where it divides into increasingly smaller air pas­sages called bronchioles. At the tip of each bronchiole there is a balloon-like cavity called an alveolus. There are about 300 million of these alveoli in each lung. The vital exchange of oxygen for carbon dioxide occurs through minute blood vessels in the thin walls of the alveoli.

You use several muscles to suck air into your lungs. The main one is the diaphragm. A dome-shaped muscular sheet attached to the lower ribs, the diaphragm divides the chest cavity from the abdomen. When the diaphragm contracts, along with other muscles between the ribs, a vacuum is created around your lungs. This causes them to expand, sucking air into the respiratory tract. When the muscles relax, the lungs contract, forcing the air back out. If this mechanism works well, you hardly notice that you are breathing. However, a numberof things can go wrong, with the lungs themselves, with essential parts of the respiratory tract, or with the muscular action.

The articles in this section are arranged in three groups.

Each group concentrates on disorders that affect one area of the respiratory system. The first group deals with problems that affect the nasal cavity and the sinuses, or air spaces behind the nose. The second group of articles deals with disorders of the throat. These include problems of both the larynx, or voice box, which is located at the top of the trachea, and the pharynx, the funnel that leads from the nasal cavity and the back of the mouth down to where the esophagus, which carries food and water to the stomach, separates from the trachea. The third group of articles deals with common diseases of the trachea, lungs and breathing mechanism.

The decision as to removal of tonsils and adenoids then rests on whether these bodies have become a menace rather than a means of protection. If an operation is felt necessary, most pediatricians prefer to hold off the tonsillectomy until a child is approximately three years of age, for if they are removed too early there is often a regrowth of the tissue. Adenoid­ectomy may be performed at any age.

Since there may be separate indica­tions for the removal of tonsils and the removal of adenoids, it is wise to consider each category separately .