Maintaining oxygen diffusion gradient in lungs

Steps:

• Recall that a steep diffusion gradient means a large concentration difference for efficient gas exchange via Fick's law.
• Identify features that keep high oxygen in alveoli versus low in blood.
• Evaluate each option for direct impact on oxygen concentration difference.
• Select the one ensuring fresh, oxygen-rich air enters alveoli continuously.

Why C is correct:

• Fresh air with high oxygen concentration (about 21%) enters alveoli, creating a steep gradient with deoxygenated blood (Fick's law: rate ∝ concentration difference).

Why the others are wrong:

• A: Increases surface area for diffusion but does not maintain the gradient itself.
• B: Aids ventilation and recoil for air renewal but not the gradient directly.
• D: Reduces diffusion distance for faster exchange but does not sustain concentration difference.

Final answer: C