Explain how the structure of a capillary makes it well suited to its function in the exchange of substances between the blood and body tissues.
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 dense networks within tissues such as muscle and the intestinal wall. They are the site where oxygen, glucose, carbon dioxide and other substances move between the blood and the body cells.
Model answer (5 marks)
Capillary walls are only one endothelial cell thick, giving a very short diffusion pathway for oxygen, glucose, CO₂ and other molecules.
The lumen of a capillary is extremely narrow, so red blood cells travel in single file. This keeps the cells and the substances they carry close to the vessel wall, further reducing the distance over which diffusion must occur.
Capillaries form dense, extensive networks within tissues, so almost every cell is adjacent to a capillary. This proximity maintains a concentration gradient and ensures efficient delivery of nutrients and removal of waste.
These structural features together make capillaries highly efficient for the exchange of substances between blood and body tissues.
The lumen of a capillary is extremely narrow, so red blood cells travel in single file. This keeps the cells and the substances they carry close to the vessel wall, further reducing the distance over which diffusion must occur.
Capillaries form dense, extensive networks within tissues, so almost every cell is adjacent to a capillary. This proximity maintains a concentration gradient and ensures efficient delivery of nutrients and removal of waste.
These structural features together make capillaries highly efficient for the exchange of substances between blood and body tissues.
Examiner tips
- Mention the one‑cell‑thick wall first to secure the key point on diffusion distance. Explain how single‑file flow reduces diffusion distance. Highlight the dense network to show maintenance of concentration gradients. Use the exact terminology: endothelial cell, lumen, diffusion pathway, concentration gradient.
Common mistakes
- Forgetting to state that the wall is one cell thick. Confusing capillary diameter with vessel diameter. Not linking the dense network to the maintenance of a concentration gradient.
Mark scheme (5 marks)
- Capillary walls are only one cell thick (have a wall that is one endothelial cell thick)
- This short/thin diffusion pathway allows substances to diffuse quickly between blood and tissues
- Capillaries have a very small lumen / narrow diameter so red blood cells pass through in single file
- This brings red blood cells (and the substances they carry) close to the capillary wall, reducing diffusion distance further / maximising exchange
- Capillaries form dense/extensive networks so that (almost) every body cell is close to a capillary, maintaining a concentration gradient / ensuring efficient supply
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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