Which feature of the alveolar structure increases the efficiency of gas exchange?

• A. Large surface area due to many alveoli.
• B. Very thick barrier that slows diffusion.
• C. Air is directly pumped by alveolar cells into the blood.
• D. Oxygen is transported by active transport across the membrane.

Answer: (D)

Gas exchange is fastest when diffusion occurs across a very large surface area and a very thin barrier. The lungs achieve this with many tiny sacs called alveoli, collectively giving an enormous surface area for gas exchange. Each alveolus has a walls that are extremely thin and is closely surrounded by capillaries, so O2 can diffuse quickly into the blood and CO2 can diffuse out into the air to be exhaled. Oxygen moves by diffusion and then binds to hemoglobin in red blood cells; it is not pumped into the blood by active transport. A thick barrier would slow diffusion, not improve it. So the feature that boosts exchange efficiency is the large surface area created by numerous alveoli.