Remember that gas exchange is what breathing is all about. It's also what COPD disrupts.

At rest, healthy people move about a half-liter (close to a half-quart) of air in and out of their lungs with each breath. They do this about 12 to 14 times a minute. (This adds up to eight liters, almost eight quarts, every minute.) As the lungs inflate, some of this air fills the larger airways. The majority fills the smaller airways and air sacs. Here in the alveoli, some of the oxygen in this fresh air and all of the excess carbon dioxide brought to the lungs by the blood are exchanged.

At rest, the right side of the heart pumps about five liters (almost five quarts) of blood through the lungs every minute. Then the blood is returned to the left side of the heart, where it is pumped throughout the body. Blood going from the heart to the lungs is called venous blood; once it leaves the lungs to return to the heart it is called arterial blood. Venous blood is oxygen poor, but rich in carbon dioxide. As venous blood passes through the lungs, oxygen is replaced and carbon dioxide removed. Then