# Explain how the structure of an alveolus is adapted to facilitate efficient gas exchange in the human lung.

> IB DP Biology Standard Level (2023 syllabus) — B3.1 Gas exchange · Explain · 4 marks

## Mark scheme (4 marks)

1. Alveoli have a very large total surface area, increasing the rate of diffusion of gases
2. Alveolar walls are one cell thick / very thin epithelium, so the diffusion distance is short, increasing the rate of diffusion
3. Alveolar walls are moist / lined with a thin film of water, allowing gases to dissolve and diffuse across the epithelium
4. Alveoli are surrounded by an extensive network of capillaries / rich blood supply, maintaining a steep concentration gradient for both oxygen and carbon dioxide

## Key terms

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

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](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-an-8edc0422) · Published by Druglandscape Ltd.