# Explain how the structure of the alveoli in the human lung is adapted to maximise the rate of gas exchange.

> IB DP Biology Higher 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 area available for diffusion of oxygen and carbon dioxide.
2. Alveolar walls consist of a single layer of squamous (type I) epithelial cells, minimising the diffusion distance for gases.
3. Alveoli are surrounded by a dense network of pulmonary capillaries, maintaining a steep concentration gradient by continuously removing oxygen and delivering carbon dioxide.
4. The surface of alveoli is coated with a thin film of moisture / surfactant, allowing gases to dissolve and diffuse across the epithelium.

## Key terms

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

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-structure-of-the-4b01b0d3) · Published by Druglandscape Ltd.