# Explain how the structure of the human lung's capillary network surrounding the alveoli facilitates efficient gas exchange.

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

## Mark scheme (4 marks)

1. Capillaries have walls that are only one cell (endothelium) thick, reducing the diffusion distance for gases.
2. The dense network of capillaries creates a large surface area in contact with the alveolar surface, increasing the rate of diffusion.
3. Continuous blood flow through the capillaries maintains a steep concentration gradient for both oxygen and carbon dioxide.
4. Red blood cells are slowed as they pass through the narrow capillaries (which are approximately the same diameter as a red blood cell), allowing more time for gas exchange to occur.

## Key terms

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

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-structure-of-the-f49e756a) · Published by Druglandscape Ltd.