# Explain how the structure of a capillary makes it well suited to its function in the exchange of substances between the blood and body tissues.

> Pearson Edexcel A-Level Biology A (9BI0) — 1.3 Mass transport in animals · Explain · 5 marks

> Capillaries are the smallest blood vessels in the body and form dense networks within tissues such as muscle and the intestinal wall. They are the site where oxygen, glucose, carbon dioxide and other substances move between the blood and the body cells.

## Mark scheme (5 marks)

1. Capillary walls are only one cell thick (have a wall that is one endothelial cell thick)
2. This short/thin diffusion pathway allows substances to diffuse quickly between blood and tissues
3. Capillaries have a very small lumen / narrow diameter so red blood cells pass through in single file
4. This brings red blood cells (and the substances they carry) close to the capillary wall, reducing diffusion distance further / maximising exchange
5. Capillaries form dense/extensive networks so that (almost) every body cell is close to a capillary, maintaining a concentration gradient / ensuring efficient supply

## Key terms

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

## 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-a-f8ce9ec7) · Published by Druglandscape Ltd.