Emphysema gradually damages the air sacs (alveoli) in lungs, making you progressively more short of breath. Emphysema is one of several diseases known collectively as chronic obstructive pulmonary disease (COPD). Smoking is the leading cause of emphysema. lungs' alveoli are clustered like bunches of grapes. In emphysema, the inner walls of the air sacs weaken and eventually rupture — creating one larger air space instead of many small ones. This reduces the surface area of the lungs and, in turn, the amount of oxygen that reaches bloodstream. When you exhale, the damaged alveoli don't work properly and old air becomes trapped, leaving no room for fresh, oxygen-rich air to enter. Treatment may slow the progression of emphysema, but it can't reverse the damage.