It occurs when the very small air sacs (called the alveoli) at the ends of the airways in the lungs start to break down from many sacs to form much bigger sacs. The alveoli are the areas of the lung where oxygen and carbon dioxide are exchanged into and out of the blood. Emphysema makes it hard for people to blow air out of the lungs because air gets trapped inside the broken alveoli due to the collapse of the walls. The trapped air in the lungs creates the characteristic "barrel chest" of emphysema.