Explain how the structure of capillaries makes them well suited for the exchange of substances between the blood and the tissues.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Capillaries have walls that are only one cell thick, which makes them very thin.
This reduces the diffusion distance so substances can move quickly between the blood and the tissues.
Capillaries form a large network and are found throughout all tissues, giving a large surface area for exchange.
Capillaries are narrow, with a small lumen, so red blood cells travel slowly and in single file, maintaining a steep concentration gradient.
Substances such as glucose, oxygen, carbon dioxide and urea diffuse down their concentration gradient between the blood and the tissues.
This reduces the diffusion distance so substances can move quickly between the blood and the tissues.
Capillaries form a large network and are found throughout all tissues, giving a large surface area for exchange.
Capillaries are narrow, with a small lumen, so red blood cells travel slowly and in single file, maintaining a steep concentration gradient.
Substances such as glucose, oxygen, carbon dioxide and urea diffuse down their concentration gradient between the blood and the tissues.
Examiner tips
- Use the exact phrases from the mark scheme (e.g. "one cell thick", "large surface area", "single file").
- Show the logical link between structure and function – e.g. thin walls → short diffusion distance.
- Include all five points in a clear, concise order.
Common mistakes
- Omitting the fact that capillaries are one cell thick.
- Failing to mention the large network and surface area.
- Not explaining the role of the small lumen and single‑file flow.
Mark scheme (5 marks)
- Capillaries have walls that are only one cell thick / very thin walls
- This reduces the diffusion distance so substances can move quickly between blood and tissues
- Capillaries form a large network / they are found throughout all tissues, giving a large surface area for exchange
- Capillaries are narrow / have a small lumen so red blood cells travel slowly / in single file, maintaining a steep concentration gradient
- Substances such as glucose, oxygen, carbon dioxide and urea can diffuse down their concentration gradient between the blood and the tissues / cells
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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