The epithelial lining of alveoli are composed of two different types of cells. Alveolar type I epithelial cells comprise about 80% of the alveolar surface area and are primarily responsible for gas exchange.[5] Alveolar type II epithelial cells play the critical roles of producing surfactant, moving water out of the airspaces, and regenerating alveolar epithelium.[5] The alveolar type II epithelial cells are more resistant to damage, so after an insult to the alveoli, most of the damage will occur to the alveolar type I epithelial cells.[5]