In the alveoli, oxygen diffuses from the alveoli into the blood because of what partial pressure relationship?

• A. Blood PO2 is higher than alveolar PO2
• B. PO2 is equal in both
• C. Diffusion is independent of PO2
• D. Alveolar PO2 is higher than blood PO2

Answer: (D)

Gas exchange relies on a partial pressure gradient: oxygen moves from areas of higher PO2 to lower PO2. In the alveoli, PO2 is about 100 mmHg, while in the surrounding pulmonary capillary blood it’s around 40 mmHg. That difference creates a strong driving force for oxygen to diffuse into the blood until the pressures equalize (as blood becomes more oxygenated). If the alveolar PO2 were lower than the blood PO2, diffusion would reverse, which isn’t the normal pattern for oxygen uptake. So the correct idea is that the alveolar PO2 is higher than the blood PO2, driving diffusion into the blood.