Alveolus << al VEE uh luhs >> most commonly refers to a tiny air sac in the lungs. The bony socket of a tooth is also called an alveolus. See Teeth (Periodontal ligament) .
The alveoli of the lungs play an important role in the process of respiration and are the site of the exchange of gases between the blood and the air. During inhalation, air enters the body through the nose. From the nose the air passes through the pharynx (the cavity behind the nose and mouth) and the larynx (the voice box). The air then enters the trachea (wind pipe), which carries air toward the lungs. Before reaching the lungs the trachea splits into two tubes called the primary bronchi. Each tube enters one lung. Within the lungs, the primary bronchi divide into smaller and smaller tubes, finally branching into extremely tiny tubes called bronchioles. The bronchioles end in hundreds of millions of thin-walled structures called alveoli. These air sacs give the lungs a tremendously extended surface area. If the alveoli were flattened out, the lungs would cover from 600 to 1,000 square feet (56 to 93 square meters).
The exchange of carbon dioxide and oxygen occurs in the alveoli. Each alveolus is surrounded by a network of small blood vessels. Like the alveoli, these small blood vessels have extremely thin walls. Blood that enters the vessels has a high level of carbon dioxide, which it has picked up from the body tissues. The blood contains little oxygen. The carbon dioxide leaves the blood and moves through the walls of the blood vessels and alveoli into the lungs. Oxygen from the air in the lungs then passes through the walls of the alveoli and blood vessels and into the blood. The blood, now rich in oxygen, leaves the lungs and travels to the heart. The heart then pumps it to cells throughout the body. The carbon dioxide is finally expelled from the lungs during exhalation.
See also Blood ; Lung ; Respiration (Breathing) .
