Explain how the structure of capillaries is adapted to allow efficient 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 of exchange between the blood and the surrounding cells.
Model answer (5 marks)
Capillaries have walls that are only one cell thick, which gives a very short diffusion distance and allows substances to move quickly between the blood and the cells.
The capillaries form extensive networks, giving a very large surface area. A larger surface area increases the rate of diffusion and allows more substances to be exchanged at once.
Because of these structural features, oxygen and glucose diffuse out of the capillaries into the surrounding cells, while waste products such as carbon dioxide diffuse from the cells into the capillaries.
The capillaries form extensive networks, giving a very large surface area. A larger surface area increases the rate of diffusion and allows more substances to be exchanged at once.
Because of these structural features, oxygen and glucose diffuse out of the capillaries into the surrounding cells, while waste products such as carbon dioxide diffuse from the cells into the capillaries.
Examiner tips
- Use the exact phrases from the mark scheme – e.g. "one cell thick", "short diffusion distance", "large surface area".
- Show the cause–effect link: structure → diffusion distance / surface area → rate of exchange.
- Mention both directions of diffusion (outward and inward).
Common mistakes
- Forgetting to state the one‑cell‑thick wall or the short diffusion distance.
- Not mentioning the large surface area or the direction of diffusion of waste products.
Mark scheme (5 marks)
- Capillaries have walls that are only one cell thick
- This short diffusion distance allows substances to diffuse quickly / efficiently between blood and cells
- Capillaries have a large surface area (due to forming extensive networks / large number of capillaries)
- A large surface area increases the rate of diffusion / allows more substances to be exchanged at once
- Substances such as glucose and oxygen diffuse out of the capillaries into cells, and waste products such as carbon dioxide diffuse in
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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