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

> Pearson Edexcel A-Level Biology A (9BI0) — Topic 4 Exchange and Transport · Explain · 5 marks

## Mark scheme (5 marks)

1. Capillaries have walls that are only one cell thick / very thin walls
2. This reduces the diffusion distance so substances can move quickly between blood and tissues
3. Capillaries form a large network / they are found throughout all tissues, giving a large surface area for exchange
4. Capillaries are narrow / have a small lumen so red blood cells travel slowly / in single file, maintaining a steep concentration gradient
5. Substances such as glucose, oxygen, carbon dioxide and urea can diffuse down their concentration gradient between the blood and the tissues / cells

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](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-capillaries-f079e089) · Published by Druglandscape Ltd.