Which statement about gaseous exchange at the alveoli is true?

• A. Oxygen diffuses from the alveolar air into the blood
• B. Alveoli do not participate in gas exchange
• C. Oxygen diffuses from the blood into the alveolar air
• D. CO2 diffuses from the alveolar air into the blood

Answer: (A)

Gas exchange in the alveoli relies on diffusion driven by differences in partial pressures between the air in the alveoli and the blood in the surrounding capillaries. Oxygen moves into the blood because the alveolar air has a higher partial pressure of oxygen than the blood arriving in the lungs. Silence from the other side of the gradient would be wrong here, as CO2 moves in the opposite direction to be expelled: it leaves the blood where its partial pressure is higher and enters the alveolar air where it is lower, to be breathed out. The alveoli do participate in this exchange, so saying they don’t participate is incorrect. The idea that oxygen would diffuse from blood into the alveolar air or that CO2 would diffuse from alveolar air into the blood contradicts the typical pressure gradients in the lungs.