The pharynx divides into two branches - the esophagus and the larynx (voice box).

The esophagus leads into the stomach and carries food to be digested. The larynx contains the vocal cords and is sorrounded by pieces of cartilage for support. The thyroid cartilage is the largest and is commonly referred to as the Adam's apple. Sounds are produced as air is expelled past the vocal cords, and the cords vibrate. The tension of the vocal cords determines the high or low pitch of the voice.

THE LARYNX

Since food entering from the mouth and air entering from the nose mix in the pharynx, even a small quantity of solid or liquid matter finding its way into the air passages could seriously block breathing and the aspirated food cold cause a chemical irritation in the lungs.

A special deterrent to this event is provided for by a flap of cartilage attached to the root of the tongue that acts like a lid over the larynx. This flap of cartilage is called the epiglottis. It covers the glottis, which is the opening to the larynx. In the act of swallowing, when food and liquid move through the throat, the epiglottis closes off the larynx, so that these cannot enter.

THE TRACHEA

On its way to the lungs, air passes from the larynx to the trachea (windpipe) - a vertical tube about 4-1/2 inches long and 1 inch in diameter. The trachea is kept open by C-shaped rings of cartilage separated by fibrous connective tissue that stiffen the front and sides of the tube.

In the region of the mediastinum, the trachea divides into two branches called bronchi (singular, bronchus). Each bronchus leads to a separate lung and divides and subdivides into smaller and finer tubes, somewhat like the branches of a tree. The smalles of the bronchial branches are called bronchioles. At the end of the bronchioles are clusters of air sacs called alveoli (singular: alveolus). Each alveolus is made of one-cell layer of epithelium. The blood that flows through the capillaries accepts the oxygen from the alveolus and deposits carbon dioxide into the alveolus to be exhaled. Oxygen is bound to a protein (hemoglobin) in erythrocytes and carried to all parts of the body.