Alveolar abnormalities are changes in the tiny air sacs in the lungs, called alveoli. The alveoli allow oxygen to enter the blood. They are very thin to let oxygen move from the lungs to the blood vessels, and for carbon dioxide to be removed from the blood vessels to the lungs.

These air sacs may collapse, fuse together, or develop thickened linings (membranes), which make it difficult for oxygen to enter the blood.