# Explain how the structure of a capillary makes it well suited to exchanging substances between the blood and body cells.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 1.3 Mass transport in animals · Explain · 5 marks

> Capillaries form dense networks throughout body tissues, linking arteries to veins. They are the site of exchange of oxygen, carbon dioxide, glucose and other substances between the blood and respiring cells.

## Mark scheme (5 marks)

1. Capillary walls are only one cell thick (a single layer of endothelial cells)
2. This short/thin diffusion pathway means substances can diffuse quickly between blood and cells
3. Capillaries have a very small lumen (narrow bore), so red blood cells are squeezed close to the wall
4. This further reduces the diffusion distance for oxygen (and other substances) from red blood cells to body cells
5. Capillaries form extensive/dense networks, giving a large surface area for exchange

## Key terms

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

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-structure-of-a-56d69c96) · Published by Druglandscape Ltd.