# Explain how carbon dioxide produced in respiring muscle cells reaches the alveoli to be breathed out.

> WJEC GCSE Biology (Wales) — 1.2.2 The respiratory system · Explain · 4 marks

## Mark scheme (4 marks)

1. Carbon dioxide diffuses out of the muscle cells into the blood (in the capillaries / bloodstream)
2. Blood carries / transports carbon dioxide to the lungs
3. Carbon dioxide diffuses from the blood (capillaries surrounding the alveoli) into the alveoli
4. Diffusion occurs because there is a higher concentration of carbon dioxide in the blood than in the alveoli (concentration gradient)

## Key terms

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

## Related

- [Revision notes for WJEC GCSE Biology (Wales)](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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