Gas Exchange

There are about 150 million alveoli in each healthy lung, providing an enormous surface area for gas exchange. Oxygen dissolves in the moisture on the inner surface of the alveoli and then diffuses across the thin-walled capillaries into the blood. Carbon dioxide in the bloodstream diffuses in the opposite direction, across the membrane of an alveolus and into the air within it. This process is illustrated in the figure at right.

Gas Exchange

The process of gas exchange in the lungs is very efficient. The air that you inhale usually contains 21 percent oxygen and 0.04 percent carbon dioxide. Exhaled air usually contains less than 15 percent oxygen and 4 percent carbon dioxide. The lungs remove about one fourth of the oxygen in the air that you inhale and increase the carbon dioxide content of that air by a factor of 100.

Human Respiration

Because oxygen dissolves easily, you may wonder why hemoglobin, the oxygen-carrying protein in blood, is needed at all. The reason is efficiency. Hemoglobin binds with so much oxygen that it increases the oxygen-carrying capacity of the blood more than 60 times. Without hemoglobin to carry the oxygen that it uses, your body might need as much as 300 liters of blood to get the same result!