Explain how the structure of a capillary wall facilitates the exchange of substances between blood and surrounding tissues.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Capillary walls are only one endothelial cell thick, which minimises the diffusion distance for substances moving between blood and tissues.
The capillary network has a very large total cross‑sectional surface area, which increases the rate of diffusion and therefore the amount of material that can be exchanged.
Pores, intercellular clefts and fenestrations in the endothelial cells allow small molecules such as glucose, amino acids, ions and water to pass by filtration or simple diffusion.
Because the total cross‑sectional area is so large, blood flow through capillaries is very slow, giving the surrounding tissues more time to take up or release substances.
The capillary network has a very large total cross‑sectional surface area, which increases the rate of diffusion and therefore the amount of material that can be exchanged.
Pores, intercellular clefts and fenestrations in the endothelial cells allow small molecules such as glucose, amino acids, ions and water to pass by filtration or simple diffusion.
Because the total cross‑sectional area is so large, blood flow through capillaries is very slow, giving the surrounding tissues more time to take up or release substances.
Examiner tips
- Use the word ‘facilitates’ and link structure to function. Mention all four key points in separate sentences. Show understanding of diffusion distance, surface area, permeability and flow rate.
Common mistakes
- Forgetting that capillaries are only one cell thick. Omitting the role of pores/clefts/fenestrations. Not mentioning the slow blood flow due to large surface area.
Mark scheme (4 marks)
- Capillary walls are only one cell (endothelial cell) thick, reducing the diffusion distance for substances moving between blood and tissues.
- The large total cross-sectional surface area of the capillary network increases the rate of diffusion / exchange of substances.
- 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.
- Slow blood flow through capillaries (due to the wide total cross-sectional area) allows more time for exchange of substances with surrounding tissues.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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