# 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.

> OCR A-Level Biology B — Advancing Biology (H422) — 2.4 Gas exchange surfaces · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. Alveoli provide a large surface area for gas exchange
2. Alveoli have thin walls / a thin membrane so diffusion distance is short
3. Alveoli are surrounded by a rich blood supply / many capillaries to maintain the concentration gradient
4. Oxygen diffuses from high concentration in the alveoli to low concentration in the blood / down the concentration gradient
5. 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

- [alveoli](https://www.gradenine.co.uk/glossary/alveoli)
- [diffusion](https://www.gradenine.co.uk/glossary/diffusion)

## Related

- [Revision notes for OCR A-Level Biology B — Advancing Biology (H422)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-marathon-runner-breathes-faster-and-57db2c42) · Published by Druglandscape Ltd.