Partial pressure gradients drive pulmonary gas exchange

Steps:

• Gases diffuse across the alveolar-capillary membrane from higher to lower partial pressure.
• Oxygen enters blood if PO₂ alveoli > PO₂ blood.
• Carbon dioxide exits blood if PCO₂ blood > PCO₂ alveoli.
• Evaluate options for gradients supporting both O₂ uptake and CO₂ elimination.

Why B is correct:

• B provides PCO₂ blood (13.9 kPa) > PCO₂ alveoli (6.3 kPa) for CO₂ diffusion to air, and PO₂ alveoli (5.3 kPa) = PO₂ blood (5.3 kPa) with no opposing gradient, per Fick's law of diffusion.

Why the others are wrong:

• A: PO₂ alveoli (6.0 kPa) < PO₂ blood (13.9 kPa), reversing O₂ flow out of blood.
• C: PCO₂ blood (5.3 kPa) < PCO₂ alveoli (6.3 kPa) and PO₂ blood (13.9 kPa) > PO₂ alveoli (5.3 kPa), reversing both flows.
• D: PO₂ alveoli (5.3 kPa) < PO₂ blood (13.9 kPa), reversing O₂ flow out of blood.

Final answer: B