What structure forms the blood-air barrier in the lungs?

• A. The alveolar-capillary membrane formed by type I pneumocytes and pulmonary capillary endothelium
• B. The alveolar septal cells
• C. The mucus layer lining airways
• D. The bronchial epithelium with cartilage

Answer: (A)

The blood-air barrier is the extremely thin diffusion surface where gas exchange occurs between air in the alveoli and blood in the pulmonary capillaries. It is formed by the alveolar epithelium (primarily the flat type I pneumocytes) and the pulmonary capillary endothelium, with their fused basement membranes creating the alveolar-capillary membrane. This structure is designed to be very thin to allow rapid diffusion of oxygen into blood and carbon dioxide out of it. The other options don’t form this barrier: mucus lining traps particles in the airways and isn’t a diffusion barrier; bronchial epithelium with cartilage is part of the conducting airways, not the gas-exchange surface; alveolar septal cells include type II pneumocytes that produce surfactant and aid repair, but the main barrier is the interface between type I pneumocytes and the capillary endothelium.