How the Lungs Work

The lungs provide a very large surface area (the size of a football field) for the exchange of oxygen and carbon dioxide between the body and the environment.

A slice of normal lung looks like a pink sponge filled with tiny bubbles or holes. These bubbles, surrounded by a fine network of tiny blood vessels, give the lungs a large surface to exchange oxygen (into the blood where it is carried throughout the body) and carbon dioxide (out of the blood). This process is called gas exchange. Healthy lungs do this very well.

Here is how normal breathing works:

• You breathe in air through your nose and mouth. The air travels down through your windpipe (trachea) then through large and small tubes in your lungs called bronchial (BRON-kee-ul) tubes. The larger tubes are bronchi (BRONK-eye), and the smaller tubes are bronchioles (BRON-kee-oles). Sometimes the word "airways" is used to refer to the various tubes or passages that air must travel through from the nose and mouth into the lungs. The airways in your lungs look something like an upside-down tree with many branches.
• At the ends of the small bronchial tubes, there are groups of tiny air sacs called alveoli. The air sacs have very thin walls, and small blood vessels called capillaries run in the walls. Oxygen passes from the air sacs into the blood in these small blood vessels. At the same time, carbon dioxide passes from the blood into the air sacs. Carbon dioxide, a normal byproduct of the body's metabolism, must be removed.

The airways and air sacs in the lung are normally elastic—that is, they try to spring back to their original shape after being stretched or filled with air, just the way a new rubber band or balloon would. This elastic quality helps retain the normal structure of the lung and helps to move the air quickly in and out. In COPD, much of the elastic quality is gone, and the airways and air sacs no longer bounce back to their original shape. This means that the airways collapse, like a floppy hose, and the air sacs tend to stay inflated. The floppy airways obstruct the airflow out of the lungs, leading to an abnormal increase in the lungs' size. In addition, the airways may become inflamed and thickened, and mucus-producing cells produce more mucus, further contributing to the difficulty of getting air out of the lungs.