Breathing

Breathing is the movement of air into and out of the lungs. Surprisingly, there are no muscles connected to the lungs. The force that drives air into the lungs comes from ordinary air pressure. How does the body use this force to inflate the lungs? The lungs are sealed in two sacs, called the pleural membranes, inside the chest cavity. At the bottom of the cavity is a large, flat muscle known as the diaphragm.

As the figure below shows, when you breathe in, or inhale, the diaphragm contracts and the rib cage rises up. This expands the volume of the chest cavity. Because the chest cavity is tightly sealed, this creates a partial vacuum inside the cavity. Atmospheric pressure does the rest, filling the lungs as air rushes into the breathing passages.

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Breathing During inhalation the rib cage rises and the diaphragm contracts, increasing the size of the chest cavity. This causes the pressure inside the lungs to decrease, and air enters.

Most of the time, exhaling is a passive event. When the rib cage lowers and the diaphragm muscle relaxes, the pressure in the chest cavity becomes greater than atmospheric pressure. Air rushes back out of the lungs. To blow out a candle, you need a greater force. Muscles surrounding the chest cavity provide that extra force, contracting vigorously just as the diaphragm relaxes.

The system works only because the chest cavity is sealed. A puncture wound to the chest—even if it does not affect the lungs directly—may allow air to leak into the chest cavity and make breathing impossible. This is one of the reasons chest wounds are always serious.

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