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 the blood loses oxygen and picks up carbon dioxide as it flows through the body's tissues, returning to the right side of the heart to be pumped through the lungs again.

Not all of the freshly inhaled air participates in this exchange of gases. Air that does not is called dead space, which includes all of the air filling the larger airways and those alveoli that aren't, for the moment, receiving any blood, and most of the air entering alveoli that temporarily have relatively little blood circulating around them. In addition, a tiny amount of blood normally does not participate effectively in this gas exchange, because it has gone to inadequately filled air sacs or has somehow bypassed the air sacs completely.

Despite this, the amount of blood passing through the lungs each minute so closely matches the amount of air reaching the alveoli each minute that a healthy person's bloodeven during exercisecarries close to the maximum amount of oxygen it can, and successfully gets rid of all the excess carbon dioxide it has picked up.

This gas exchange process is so efficient because of the incredible expanse actually covered by the thin, blood vessel-filled membrane that forms the walls of the lungs' 300 million air sacs. It is this membrane through which gas exchange occurs. If this membrane could be stretched out flat, it would cover a tennis court!

Obstructed Airways Hurt

But seriously obstructed airways disrupt this matchup. Air entering the lungs of a chronic bronchitis patientone who has a substantial number of airways plugged with mucusis very unevenly distributed. Much of it goes to the unobstructed areas of the lungs. But the blood coming from the heart to pick up oxygen and shed carbon dioxide is not redistributed to anywhere near the same degree. Too much of it goes to obstructed areas of the lungs, the areas that cannot fully oxygenate it or adequately remove the carbon dioxide it carries. The result is a ventilation-blood flow imbalance.

The patient becomes chronically hypoxicthat is, the level of oxygen in his blood regularly falls below what it should be. Then his body's musclesincluding his respiratory musclescannot get the amount of oxygen they need to function effectively. The level of carbon dioxide in his blood rises above what it should be, adding to his sense of fatigue.

Lungs that have lost a substantial part of the gas exchange membrane which is the basis of emphysemacan no longer accept anywhere near eight liters of air and five liters of blood each minute. Gas exchange can no longer occur fast enough to satisfy the body's needs.