# Explain how the structure of a capillary is adapted to facilitate the exchange of substances between blood and body 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, providing a short diffusion distance for substances to move between blood and tissues.
2. The narrow lumen forces red blood cells to travel slowly and in single file, increasing the time available for diffusion / gas exchange.
3. Capillaries form dense networks / beds that increase the surface area available for exchange with surrounding cells.
4. Small pores / gaps between endothelial cells allow small molecules such as glucose, oxygen, and carbon dioxide to pass freely by diffusion or filtration.

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-structure-of-a-48d060b8) · Published by Druglandscape Ltd.