# A long-distance runner finishes a race and continues to breathe heavily for several minutes afterwards. During the race, the runner's muscles produced lactic acid. Explain why lactic acid is produced during intense exercise and describe what happens to the lactic acid after the race has finished.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 7 Run for your Life · Explain · 5 marks

> During intense exercise, the demand for energy in muscle cells can exceed the rate at which oxygen can be delivered to those cells.

## Mark scheme (5 marks)

1. During intense exercise, oxygen cannot be delivered to muscles fast enough / muscles respire faster than oxygen can be supplied
2. Muscles respire anaerobically / anaerobic respiration occurs
3. Anaerobic respiration produces lactic acid (from the incomplete breakdown of glucose)
4. After exercise, lactic acid is transported (in the blood) to the liver
5. In the liver, lactic acid is converted back to glucose / oxidised / broken down using the oxygen debt

## Key terms

- [lactic acid](https://www.gradenine.co.uk/glossary/lactic-acid)

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](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-long-distance-runner-finishes-a-race-c7fc6d8f) · Published by Druglandscape Ltd.