The alveolar-capillary membrane tends to

• A. Decrease in thickness
• B. Decrease in function
• C. Thickens
• D. Thins

Answer: (C)

Gas exchange relies on a very thin alveolar-capillary barrier. As people age, structural changes such as increased connective tissue in the interstitium and thickening of the basement membranes around the alveoli and capillaries make this barrier thicker. This greater diffusion distance slows the transfer of gases, leading to reduced diffusion capacity—especially during exercise. So the alveolar-capillary membrane tends to thicken. A thinner barrier would enhance diffusion, not hinder it, and a decrease in function is a consequence of the thicker barrier rather than its tendency.