A marathon runner breathes faster and more deeply during a race than when resting. Explain how the features of alveoli allow oxygen to move into the blood efficiently, and describe what happens to the rate of diffusion when the runner is exercising compared to when they are at rest.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During a marathon, a runner's muscles demand a greatly increased supply of oxygen. The lungs must exchange gases as efficiently as possible to meet this demand.
Model answer (5 marks)
Alveoli provide a large surface area for gas exchange, allowing many molecules of O₂ to reach the blood at once.
Alveoli have very thin walls (≈0.5 µm) so the diffusion distance is short.
They are surrounded by a rich capillary network, keeping the concentration of O₂ in the blood low and maintaining a strong gradient.
O₂ diffuses from the high concentration in the alveolar air to the low concentration in the capillary blood, down the concentration gradient.
During exercise the muscle demand lowers the O₂ concentration in the blood, steepening the gradient and so the rate of diffusion increases.
Alveoli have very thin walls (≈0.5 µm) so the diffusion distance is short.
They are surrounded by a rich capillary network, keeping the concentration of O₂ in the blood low and maintaining a strong gradient.
O₂ diffuses from the high concentration in the alveolar air to the low concentration in the capillary blood, down the concentration gradient.
During exercise the muscle demand lowers the O₂ concentration in the blood, steepening the gradient and so the rate of diffusion increases.
Examiner tips
- Use the exact terms: surface area, thin walls, capillary network, concentration gradient, diffusion rate.
- Show the cause–effect chain: larger surface area + thinner wall + rich blood supply → efficient diffusion; exercise → steeper gradient → higher diffusion rate.
- Keep the answer concise – 5 points, one sentence per point.
- Mention the direction of diffusion (alveoli to blood).
Common mistakes
- Confusing diffusion distance with diffusion rate; forgetting to mention the concentration gradient.
- Using vague terms like "more oxygen" instead of "steeper gradient" or "higher diffusion rate”.
- Omitting the role of the capillary network or the thin alveolar wall.
Mark scheme (5 marks)
- Alveoli provide a large surface area for gas exchange
- Alveoli have thin walls / a thin membrane so diffusion distance is short
- Alveoli are surrounded by a rich blood supply / many capillaries to maintain the concentration gradient
- Oxygen diffuses from high concentration in the alveoli to low concentration in the blood / down the concentration gradient
- During exercise the concentration gradient is steeper / larger, so the rate of diffusion increases (because muscles use oxygen faster, lowering blood oxygen concentration)
Key terms in this question
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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