Answer: Simple diffusion along a pressure gradient.
- A Active transport across alveolar walls
- B Facilitated transport using carrier proteins
- C Bulk flow driven by blood pressure
- D Simple diffusion along a pressure gradient
Correct answer: D. Simple diffusion along a pressure gradient
Explanation: Gases move across the alveolar-capillary membrane by simple diffusion, driven by differences in their partial pressures on either side.
The S-shaped (sigmoid) curve reflects cooperative binding between Hb's four O₂ sites; the Bohr effect shifts the whole curve to the right under conditions found in actively respiring tissue (more CO₂, more acid, more heat), making haemoglobin release oxygen more readily exactly where it's needed.
Concept context
The mechanics of breathing, lung volumes, gas transport in blood, the Bohr effect, and respiratory regulation.