# Explain how the structure of capillaries is adapted to allow the exchange of substances between the blood and body cells.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 1.2 Heart, blood and blood vessels · Explain · 5 marks

> Capillaries are the smallest blood vessels in the body and form extensive networks within tissues. They are the site where oxygen, glucose and waste products are exchanged between the blood and body cells.

## Mark scheme (5 marks)

1. Capillary walls are only one cell thick / a single layer of cells (endothelium)
2. Short diffusion distance (as a result of thin walls), so substances can diffuse quickly/easily between blood and cells
3. Capillaries have a very small diameter / narrow lumen, so red blood cells pass through in single file
4. Capillaries form extensive/large networks within tissues, increasing the surface area for exchange
5. Substances (e.g. oxygen, glucose) move out of the blood by diffusion down a concentration gradient into the cells; waste products (e.g. carbon dioxide) diffuse in the opposite direction

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- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](https://www.gradenine.co.uk/learn)
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