A-Level Biology May/June 2024 Q3(b)(ii): Suggest and explain how a steep oxygen concentration gradient is maintained in the lungs.

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  A steep oxygen concentration gradient between the alveolar air and the blood in the capillaries is maintained by several coordinated physiological processes:

  1. Constant ventilation of the lungs ensures that the air within the alveoli is continuously replenished. Inhalation brings in fresh air with a high partial pressure of oxygen, replacing the air from which oxygen has diffused into the blood. This keeps the oxygen concentration in the alveoli consistently high.

  2. The blood arriving at the lungs via the pulmonary artery is deoxygenated. Having returned from respiring body tissues where oxygen was used, this blood has a very low partial pressure of oxygen. This establishes the 'low' end of the concentration gradient.

  3. As oxygen diffuses across the alveolar and capillary walls into the blood plasma, it is rapidly removed from the solution by binding to haemoglobin in red blood cells, forming oxyhaemoglobin. This keeps the concentration of free oxygen in the blood plasma low, ensuring the gradient for diffusion from the alveoli is maintained.

  4. The continuous flow of blood through the dense capillary network surrounding the alveoli means that as soon as blood becomes oxygenated, it is transported away from the lungs. It is constantly replaced by more deoxygenated blood, which perpetuates the steep gradient.

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  How the marks are awarded

  • B1 — Explaining that ventilation or breathing constantly brings oxygen-rich air into the alveoli, maintaining a high concentration there.
  • B1 — Stating that the blood arriving in the alveolar capillaries is deoxygenated, meaning it has a low partial pressure of oxygen.
  • B1 — Describing how oxygen is taken up by haemoglobin, which keeps the oxygen concentration in the blood plasma low and maintains the gradient for diffusion.
  • B1 — Explaining that the continuous flow of blood removes the newly oxygenated blood and replaces it with deoxygenated blood.

  Common mistakes

  • Describing features that increase the rate of diffusion (e.g., thin walls, large surface area) instead of explaining how the concentration gradient itself is maintained.
  • Forgetting to explain the role of haemoglobin; simply stating that oxygen diffuses into the blood does not explain why the blood's oxygen concentration remains low.
  • Only explaining one side of the gradient, for example, discussing ventilation to keep alveolar oxygen high but failing to explain why blood oxygen is kept low.
  • Giving a vague answer like 'blood flows to the lungs' without specifying that it is deoxygenated blood arriving and oxygenated blood being removed.

  Examiner tip: For questions about 'maintaining a gradient', always explain the dynamic processes that actively keep the 'high' side high and the 'low' side low.

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