Maintaining diffusion gradients via ventilation and perfusion

Steps:

• Steep gradients require constant renewal of O2 in alveoli and low O2 in incoming blood for diffusion.
• Elastic recoil in alveoli during exhalation expels CO2-rich air, allowing fresh O2 intake.
• Pulmonary artery delivers deoxygenated blood, ensuring continuous low O2 levels for uptake.
• Both processes sustain gradients by preventing equilibrium.

Why A is correct:

• Elastic recoil facilitates ventilation to refresh alveolar gases; pulmonary artery supplies deoxygenated blood to maintain low pulmonary O2, per Fick's law of diffusion.

Why the others are wrong:

• B: Incorrectly denies both mechanisms' roles in gradient maintenance.
• C: Wrongly dismisses elastic recoil's ventilation aid.
• D: Erroneously rejects pulmonary artery's perfusion contribution.

Final answer: A