Which epithelial type lines the alveolar sacs for gas exchange?

• A. Simple cuboidal epithelium lining alveolar sacs.
• B. Simple squamous epithelium lining the alveolar sacs (Type I pneumocytes) for diffusion.
• C. Stratified squamous epithelium lining alveoli.
• D. Pseudostratified ciliated columnar epithelium in alveoli.

Answer: (B)

Gas exchange relies on a very thin barrier between air and blood. The alveolar walls are lined by a single layer of flat cells—simple squamous epithelium. In the alveoli, these cells, called Type I pneumocytes, form most of the surface and minimize the distance gases must travel to cross into the capillaries. This thin sheet, together with the capillary endothelium, makes the respiratory membrane essential for efficient diffusion of oxygen into the blood and carbon dioxide out of it. There are also Type II pneumocytes that are more cuboidal and secrete surfactant to reduce surface tension and assist repair, but they are not the primary lining for gas exchange. Other epithelial types—cuboidal, stratified, or pseudostratified ciliated columnar—are found in other parts of the respiratory tract and would add thickness or misplace the lining for the alveoli.