Normal and emphysemic alveoli. Computer illustration comparing healthy alveoli (left) with those from a lung affected by emphysema (right). Alveoli are the tiny air sacs of the lungs where gases are exchanged between the air in the lungs and the blood. Emphysema is a lung disease often caused by tobacco smoking. It causes damage to the alveoli, preventing them from taking the shape necessary for inhalation and breathing. This means air cannot get in and old air cannot get out from the damaged air sac, reducing gas exchange in the lungs and causing breathing difficulty.