Which statement describes the diffusion principle for oxygen in the lungs?

• A. Oxygen diffuses from pulmonary capillary blood into alveolar air.
• B. Oxygen diffuses from alveolar air into pulmonary capillary blood along its partial pressure gradient until equilibrated.
• C. Oxygen diffusion occurs only with active transport.
• D. Oxygen diffusion occurs independently of partial pressure.

Answer: (B)

Oxygen diffusion in the lungs is driven by a partial pressure gradient across the alveolar–capillary membrane. Alveolar air has a higher PO2 than the blood in the pulmonary capillaries, so oxygen moves from air into blood. It continues until the blood PO2 nearly equals the alveolar PO2, reaching equilibrium. This is exactly what the statement describes: diffusion from alveolar air into pulmonary capillary blood along its partial pressure gradient until equilibrated. Diffusion here is a passive process, not requiring active transport, and it depends on a gradient in partial pressures.