The airways and air sacs in the lung are normally elastic, meaning 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.
In the type of COPD called emphysema, the walls between the air sacs are destroyed, leading to a few large air sacs, instead of many tiny ones. Then, the lung looks like a sponge with large bubbles or holes in it instead of a sponge with very even tiny holes. These few large air sacs have less surface area than the many tiny ones for the exchange of oxygen and carbon dioxide.