Maintaining CO2 Diffusion Gradient in Alveoli Steps:

• Identify the diffusion gradient as the partial pressure difference of CO2 between blood and alveoli.
• Recognize that ventilation removes CO2 from alveoli, lowering alveolar PCO2.
• Note that pulmonary blood flow delivers CO2-rich blood to alveoli, raising blood PCO2.
• Confirm factors 1 (ventilation) and 2 (blood flow) sustain this gradient; factor 3 is irrelevant.

Why A is correct:

• Fick's law of diffusion states rate depends on concentration gradient, maintained by ventilation (factor 1) and perfusion (factor 2) ensuring constant PCO2 difference.

Why the others are wrong:

• B includes irrelevant factor 3, ignoring blood flow's role.
• C omits ventilation, which is essential for gradient renewal.
• D excludes ventilation, allowing alveolar CO2 to equilibrate without removal.

Not enough information on exact factors 1–3. Final answer: A