In the life-supporting process of aerobic respiration, oxygen from incoming air enters the blood; and carbon dioxide, a waste gas from the metabolism of food, is exhaled into the atmosphere. Air entering the lungs contains about 21 percent oxygen and 0.04 percent carbon dioxide. Air leaving the lungs contains about 14 percent oxygen and about 4.4 percent carbon dioxide. The composition of the air changes between inspiration and expiration, when the air is deep in the lung tissue.
The exchange of gases takes place when air reaches the alveoli. These small sacs are only one cell thick, and they are surrounded by blood capillaries that are also only one cell thick. Air diffuses through these cells into the capillary blood, which carries the oxygen-rich air to the heart to be distributed throughout the body. In the alveoli, at the same time, gaseous carbon dioxide diffuses from the blood into the lung and is expired.