Respiratory System: A Quick Look At How It Works
Of all the substances needed to sustain life, oxygen -- an odorless, colorless, and tasteless gas -- is perhaps the most critical because it is essential for all stages of metabolism, the various biochemical functions that maintain the body. Without oxygen, cells begin to die within minutes.
With each breath, oxygen is taken into the lungs and carbon dioxide and other wastes are expelled. Although you can deliberately hold your breath for a short period, breathing actually is an automatic process controlled by the brain's respiratory center. When performing quiet activities, a person takes about 14 breaths a minute, but the respiration rate may be slower during sleep or meditation and higher during exercise or other activities that demand extra oxygen.
Air is inhaled through the nose or mouth and passes through the larynx, or voice box, into the trachea, or the windpipe, and then to the bronchi and bronchioles, air tubes that branch off the trachea. These tubes are lined with millions of cilia, hairlike strands that beat rhythmically to keep dust, germs, and other airborne particles out of the lungs. The cilia also help clear the lungs of mucus produced by the mucous cells lining the bronchial tubes.
The bronchioles terminate in clusters of alveoli, tiny, balloonlike air sacs that are responsible for ensuring that the blood has a steady supply of fresh oxygen. Oxygen exhange takes pace on the surface of the lung's 700 million or so alveoli, which, if flattened out, would almost cover a tennis court. The air sacs are elastic, expanding during inhalation and deflating partially as air is exhaled. If alveoli lose their elasticity, as is the case in emphysema, stale air becomes trapped in the sacs and the body becomes starved for oxygen.