Alveoli refer to final branching of the respiratory tree which acts as the primary gas exchange units of the lung. The gas-blood barrier that lies between the pulmonary capillaries and the alveolar space is very thin which paves way for rapid gas exchange. Oxygen needs to diffuse through the alveolar epithelium and the capillary endothelium to reach the blood, while the carbon dioxide takes the reverse pathway to reach the alveoli. Alveolar epithelial cells are of two prime kinds – Type I cells and Type II cells. Type I cells comprise cytoplasmic extensions that spread across sleekly with the alveolar walls, and the alveolar epithelium. Type II cells are more solid and releases surfactant that lines the alveoli and contributes to alveolar stability.