Explain how the structure of a capillary is adapted to facilitate the exchange of substances between blood and body 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, giving a very short diffusion distance for substances to move between blood and tissues.
The narrow lumen forces red blood cells to travel slowly and in single file, which increases the time available for diffusion and gas exchange.
Capillaries form dense networks or beds, thereby increasing the surface area available for exchange with surrounding cells.
Small pores or gaps between endothelial cells allow small molecules such as glucose, oxygen and carbon dioxide to pass freely by diffusion or filtration.
The narrow lumen forces red blood cells to travel slowly and in single file, which increases the time available for diffusion and gas exchange.
Capillaries form dense networks or beds, thereby increasing the surface area available for exchange with surrounding cells.
Small pores or gaps between endothelial cells allow small molecules such as glucose, oxygen and carbon dioxide to pass freely by diffusion or filtration.
Examiner tips
- Use the exact wording from the mark scheme (e.g., "only one endothelial cell thick").
- Show the four points in separate sentences or bullet‑style points.
- Mention both diffusion and filtration where appropriate.
Common mistakes
- Confusing capillaries with arterioles or venules; not noting the single‑cell thickness.
- Omitting the slow, single‑file flow of red cells or the dense network of capillaries.
- Using vague terms like "thin walls" instead of "one cell thick".
Mark scheme (4 marks)
- Capillary walls are only one cell (endothelial cell) thick, providing a short diffusion distance for substances to move between blood and tissues.
- The narrow lumen forces red blood cells to travel slowly and in single file, increasing the time available for diffusion / gas exchange.
- Capillaries form dense networks / beds that increase the surface area available for exchange with surrounding cells.
- Small pores / gaps between endothelial cells allow small molecules such as glucose, oxygen, and carbon dioxide to pass freely by diffusion or filtration.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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