What composes the blood-air barrier in the alveoli?

• A. Thick epithelial layer and alveolar macrophages
• B. Thin alveolar epithelium (mostly Type I pneumocytes), fused basement membranes, and capillary endothelium
• C. Alveolar cartilage and smooth muscle
• D. Surfactant and mucus layers

Answer: (B)

The blood–air barrier in the alveoli is formed by a very thin alveolar epithelium, the adjacent fused basement membranes, and the capillary endothelium. The thin epithelial layer is mainly Type I pneumocytes, which cover most of the alveolar surface and provide the minimal distance for gas diffusion. The basement membranes of the alveolar epithelium and the capillary endothelium merge into one thin barrier, keeping the diffusion path brief. The capillary endothelium lines the surrounding blood vessels and completes the barrier between air and blood. Alveolar macrophages roam in the air space for defense, but they are not part of the barrier itself. Alveolar cartilage or smooth muscle aren’t present in the alveoli, and surfactant is a surface-coating substance rather than a structural part of the barrier.