Explain how the structure of capillaries is adapted to allow the exchange of substances between the blood and body cells.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Capillaries are the smallest blood vessels in the body and form extensive networks within tissues. They are the site where oxygen, glucose and waste products are exchanged between the blood and body cells.
Model answer (5 marks)
Capillary walls are only one cell thick, consisting of a single layer of endothelial cells, which gives a very short diffusion distance for oxygen, glucose and waste products.
The walls are extremely thin, so substances can diffuse quickly and easily between the blood and the surrounding tissue cells.
Capillaries have a very small diameter; the lumen is narrow enough that red blood cells must pass through in single file, allowing the blood to be in close contact with the tissue.
Capillaries form extensive networks within tissues, greatly increasing the surface area available for exchange.
Oxygen and glucose move out of the blood by diffusion down a concentration gradient into the cells, while waste products such as carbon dioxide diffuse in the opposite direction from the cells into the blood.
The walls are extremely thin, so substances can diffuse quickly and easily between the blood and the surrounding tissue cells.
Capillaries have a very small diameter; the lumen is narrow enough that red blood cells must pass through in single file, allowing the blood to be in close contact with the tissue.
Capillaries form extensive networks within tissues, greatly increasing the surface area available for exchange.
Oxygen and glucose move out of the blood by diffusion down a concentration gradient into the cells, while waste products such as carbon dioxide diffuse in the opposite direction from the cells into the blood.
Examiner tips
- Use the exact terms: one‑cell thick, single layer of endothelium, short diffusion distance, narrow lumen, extensive networks, diffusion down a concentration gradient.
- Show the direction of diffusion for both nutrients and waste.
- Keep the answer concise – 5 marks, so one sentence per point.
- Mention the role of the thin wall in reducing diffusion distance.
Common mistakes
- Writing that capillaries are thick or that they have a wide lumen.
- Forgetting to state the direction of diffusion for waste products.
- Using vague phrases like "allow exchange" without explaining how the structure achieves it.
Mark scheme (5 marks)
- Capillary walls are only one cell thick / a single layer of cells (endothelium)
- Short diffusion distance (as a result of thin walls), so substances can diffuse quickly/easily between blood and cells
- Capillaries have a very small diameter / narrow lumen, so red blood cells pass through in single file
- Capillaries form extensive/large networks within tissues, increasing the surface area for exchange
- Substances (e.g. oxygen, glucose) move out of the blood by diffusion down a concentration gradient into the cells; waste products (e.g. carbon dioxide) diffuse in the opposite direction
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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