What is the blood gas barrier?

• A. a barrier that prevents gas exchange
• B. an extremely thin barrier between the alveoli & capillary blood
• C. the membrane surrounding the heart
• D. a barrier that lines the trachea

Answer: (B)

Gas exchange relies on an extremely thin interface between the air in the alveoli and the blood in the surrounding capillaries. This blood gas barrier, or alveolar-capillary membrane, is an exceptionally thin barrier that oxygen and carbon dioxide diffuse across. It is formed by the alveolar lining (mostly type I pneumocytes), the capillary endothelium, and their fused basement membranes, with only a tiny interstitial space separating them. Because of this great thinness, diffusion distance is minimized and gas transfer happens rapidly, allowing oxygen to move into the blood and carbon dioxide to move into the alveolar air efficiently. If this barrier thickens due to disease, diffusion slows and gas exchange becomes impaired. It is not the membrane around the heart, nor the lining of the trachea, and it does not serve to block gas exchange.