Explain how the structure of a capillary is adapted to allow 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, so the diffusion distance for oxygen, glucose, CO₂ and waste products is very short.
The lumen is very narrow, forcing red blood cells to move in single file and slowing the flow; this increases the time available for exchange.
Capillaries form dense, highly branched networks, giving a large surface area for contact with tissue fluid.
Small pores or gaps between adjacent endothelial cells allow small solutes to pass by diffusion or filtration into the surrounding tissue.
The lumen is very narrow, forcing red blood cells to move in single file and slowing the flow; this increases the time available for exchange.
Capillaries form dense, highly branched networks, giving a large surface area for contact with tissue fluid.
Small pores or gaps between adjacent endothelial cells allow small solutes to pass by diffusion or filtration into the surrounding tissue.
Examiner tips
- Mention the single‑cell thickness first – it’s the key structural adaptation. Highlight the slow flow and single‑file RBCs as a functional consequence. Show that the large surface area comes from dense branching. Include the role of endothelial gaps for solute passage.
Common mistakes
- Forgetting to state the single‑cell thickness. Mixing up the function of the narrow lumen (e.g. saying it increases flow instead of slowing it). Omitting the importance of the dense network for 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 tissue fluid.
- The narrow lumen forces red blood cells to travel in single file / slows blood flow, increasing the time available for exchange.
- Capillaries form dense networks / branching beds that greatly increase the surface area available for exchange with tissues.
- Pores / gaps between adjacent endothelial cells allow small solutes (e.g. glucose, oxygen, carbon dioxide, waste products) to pass by diffusion or filtration into tissue fluid.
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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