# Explain how the structure of alveoli makes them well adapted as an exchange surface for gaseous exchange in the lungs.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 8 Grey Matter · Explain · 5 marks

> The lungs contain millions of tiny air sacs called alveoli. Oxygen must move from the air inside the alveoli into the blood, while carbon dioxide moves in the opposite direction.

## Mark scheme (5 marks)

1. Large surface area (of alveoli) increases the rate of diffusion / allows more gas to be exchanged at once
2. Thin walls reduce the diffusion distance so gases can diffuse across more quickly
3. Good/rich blood supply maintains a steep concentration gradient (for oxygen / carbon dioxide)
4. Moist lining allows gases to dissolve so they can diffuse across the membrane
5. Overall, these adaptations maximise the rate of diffusion of oxygen into the blood (and carbon dioxide out), meeting the demands of aerobic respiration

## Key terms

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

## 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/explain-how-the-structure-of-alveoli-fd0fc4c4) · Published by Druglandscape Ltd.