A disorder affecting the alveoli (tiny air sacs) of the lungs. The transfer of oxygen and carbon dioxide in the lungs takes place in the walls of the alveoli. In emphysema, the alveoli become abnormally inflated, damaging their walls and making it harder to breathe. People who smoke or have chronic bronchitis have an increased risk of emphysema. Emphysema is a type of chronic obstructive pulmonary disease (COPD).