# Explain how the structure of a capillary wall facilitates the exchange of substances between blood and surrounding tissues.

> IB DP Biology Higher 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 tissues.
2. The large total cross-sectional surface area of the capillary network increases the rate of diffusion / exchange of substances.
3. Pores / intercellular clefts / fenestrations between or through endothelial cells allow passage of small molecules (e.g. glucose, amino acids, ions, water) by filtration or diffusion.
4. Slow blood flow through capillaries (due to the wide total cross-sectional area) allows more time for exchange of substances with surrounding tissues.

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