Which epithelium forms the air–blood barrier in the alveoli?

• A. The alveolar epithelium, primarily the thin simple squamous Type I pneumocytes, with capillary endothelium.
• B. The pseudostratified ciliated columnar epithelium
• C. Stratified squamous epithelium
• D. Simple cuboidal epithelium

Answer: (A)

Gas exchange across the alveolar wall happens across a very thin barrier formed by the alveolar epithelium in close contact with the capillary endothelium. The surface is lined mainly by extremely thin, flat Type I pneumocytes (simple squamous) that cover most of the alveolar surface and provide the primary diffusion pathway for gases. The basement membranes of the Type I cells and the adjacent endothelial cells fuse, creating a minimal, single barrier between air and blood. Interspersed among these are Type II pneumocytes, which are cuboidal and secrete surfactant to reduce surface tension and help repair the epithelium, but they do not form the main gas‑exchange barrier. The capillary endothelium itself is also a thin, crucial component of this barrier. Choices describing pseudostratified ciliated columnar or stratified squamous epithelium refer to airways or protective surfaces outside the alveoli, and simple cuboidal epithelium corresponds to Type II cells rather than the thin barrier formed by Type I cells.