Ventilation and perfusion sustain gas exchange gradients Steps:

• Identify that gas exchange relies on diffusion driven by partial pressure gradients of O2 and CO2.
• Recognize ventilation renews alveolar air, keeping high O2 and low CO2 levels.
• Note perfusion circulates blood, removing O2 and adding CO2 to maintain disequilibrium.
• Eliminate option 1 (e.g., membrane thickness) as it facilitates diffusion but doesn't sustain gradients.

Why D is correct:

• Fick's law of diffusion states rate ∝ concentration gradient; ventilation (2) and perfusion (3) continuously refresh and deplete gases to preserve steep gradients.

Why the others are wrong:

• A includes 1, which affects diffusion distance but not gradient maintenance.
• B omits 3, ignoring blood flow's role in removing exchanged gases.
• C omits 2, neglecting fresh air's necessity to replenish alveolar O2.

Final answer: D