# Explain how the structure of the alveoli is adapted to allow efficient gas exchange in the lungs.

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

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

## Mark scheme (5 marks)

1. Alveoli provide a large surface area
2. Large surface area increases the rate of diffusion (of gases)
3. Alveoli walls are very thin / one cell thick
4. Thin walls reduce the diffusion distance so gases can pass across quickly
5. Alveoli have a good blood supply / rich capillary network, which maintains a steep concentration gradient

## Key terms

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

## 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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-structure-of-the-720ee540) · Published by Druglandscape Ltd.