# Explain how the structure of capillaries makes them well adapted for the exchange of substances between the blood and the tissues.

> Cambridge International IGCSE Biology (0610) — 9.3 Blood vessels · Explain · 4 marks

## Mark scheme (4 marks)

1. Capillaries have walls that are only one cell thick, which decreases the diffusion distance for substances moving between the blood and tissues.
2. Capillaries have a narrow lumen / narrow diameter, which also reduces the diffusion distance between the blood and surrounding cells.
3. Capillaries are highly branched and have a large surface area, increasing the area over which exchange can occur.
4. Blood flows slowly through capillaries, allowing more time for substances such as oxygen and carbon dioxide to diffuse between the blood and tissues.

## Key terms

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

## Related

- [Revision notes for Cambridge International IGCSE Biology (0610)](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-capillaries-ce308cfe) · Published by Druglandscape Ltd.