Emphysema is a disease in which alveoli become damaged. They stretch out of shape and often rupture.
When an air sac ruptures, air escapes into surrounding tissue. This forms a bubble within the lung. Doctors call it an emphysematous bleb. Scar tissue forms around the bleb. The bleb no longer has the ability to perform gas exchange.
If there is rupture of multiple alveoli that are next to each other, the bleb gets large enough to be seen on a chest X-ray. Large blebs are usually called bullae (pronounced bull-i).
The damaged alveoli from emphysema cannot be repaired. Smoking causes 90% of emphysema cases. Because it is not reversible, anyone with emphysema must quit smoking to try and slow the further destruction of the remaining alveoli.