Physiology of Breathing

The second stage of breathing, blowing air out of the lungs, is called expiration or exhalation. After the oxygen and carbon dioxide trade places in the alveoli, the diaphragm relaxes and positive pressure is restored to the chest cavity. This forces the used air out of the lungs, following the reverse of the path that it used to get in the lungs. The entire breathing process is repeated 10 to 20 times per minute in a healthy adult.