AQA A-Level Biology 2025/6

Biology • A-Level

Question 1 of 41

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Describe how the structure of the respiratory membrane and the characteristics of the pulmonary circulation work together to ensure rapid oxygen uptake and carbon‑dioxide removal during a single breath.

Hint: Consider surface area, wall thickness, partial‑pressure differences and the role of blood flow and haemoglobin.

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Correct answer: Oxygen diffuses from the alveoli into the blood because the respiratory membrane is extremely thin, highly vascularised and has a large surface area, creating a steep partial‑pressure gradient. Pulmonary capillaries, which receive the entire cardiac output, bring deoxygenated blood into close contact with this membrane, allowing rapid uptake. Simultaneously, carbon‑dioxide diffuses in the opposite direction down its own gradient and is carried away in the plasma and bound to haemoglobin, ready to be expelled by the circulatory system. The continuous flow of blood maintains these gradients and ensures efficient gas exchange.

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Oxygen diffuses from the alveoli into the blood because the respiratory membrane is extremely thin, highly vascularised and has a large surface area, creating a steep partial‑pressure gradient. Pulmonary capillaries, which receive the entire cardiac output, bring deoxygenated blood into close contact with this membrane, allowing rapid uptake. Simultaneously, carbon‑dioxide diffuses in the opposite direction down its own gradient and is carried away in the plasma and bound to haemoglobin, ready to be expelled by the circulatory system. The continuous flow of blood maintains these gradients and ensures efficient gas exchange.

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