# Explain how the structure of a capillary is adapted to allow the exchange of substances between blood and surrounding tissues.

> IB DP Biology Standard Level (2023 syllabus) — B3.2 Transport · Explain · 4 marks

## Mark scheme (4 marks)

1. Capillary walls are only one cell (endothelial cell) thick, reducing the diffusion distance for substances moving between blood and tissue fluid.
2. The narrow lumen forces red blood cells to travel in single file / slows blood flow, increasing the time available for exchange.
3. Capillaries form dense networks / branching beds that greatly increase the surface area available for exchange with tissues.
4. Pores / gaps between adjacent endothelial cells allow small solutes (e.g. glucose, oxygen, carbon dioxide, waste products) to pass by diffusion or filtration into tissue fluid.

## Key terms

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

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](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-a-06041870) · Published by Druglandscape Ltd.