According to the Heart, Lung, and Blood Institute, Chronic Obstructive Pulmonary Disease (COPD) is a lung disease in which the lungs are damaged, making it hard to breathe. In COPD, the airways—the tubes that carry air in and out of your lungs—are partly obstructed, making it difficult to get air in and out. Cigarette smoking is the most common cause of COPD. Most people with COPD are smokers or former smokers. Breathing in other kinds of lung irritants, like pollution, dust, or chemicals, over a long period of time may also cause or contribute to COPD.
The airways branch out like an upside-down tree, and at the end of each branch are many small, balloon-like air sacs called alveoli (al-VEE-uhl-EYE). In healthy people, each airway is clear and open. The air sacs are small and dainty, and both the airways and air sacs are elastic and springy. When you breathe in, each air sac fills up with air like a small balloon; when you breathe out, the balloon deflates and the air goes out. In COPD, the airways and air sacs lose their shape and become floppy. Less air gets in and less air goes out.