# A sports scientist is studying the effects of exercise intensity on respiration in athletes. During a high-intensity sprint, an athlete's muscles work so hard that oxygen cannot be delivered quickly enough to meet demand. Explain what happens to respiration in the athlete's muscles during and immediately after the sprint, and why the athlete continues to breathe heavily for several minutes after stopping.

> OCR A-Level Biology B — Advancing Biology (H422) — 4.2 Respiration · Explain · 5 marks

> During a 400 m sprint, an athlete's breathing rate and heart rate increase dramatically. Even after crossing the finish line, the athlete remains bent over, breathing deeply for several minutes.

## Mark scheme (5 marks)

1. During the sprint, muscles switch to anaerobic respiration (because oxygen supply is insufficient)
2. Anaerobic respiration produces lactic acid (in addition to releasing energy from glucose)
3. Lactic acid causes muscle fatigue / pain / cramp (which limits performance)
4. After the sprint, the athlete continues to breathe heavily to take in extra oxygen to break down / oxidise the lactic acid (repaying the oxygen debt)
5. The lactic acid is transported (in the blood) to the liver where it is converted back to glucose

## 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)
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