Explain how the structure of a capillary makes it well suited to exchanging substances between the blood and body cells.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 1.3 Mass transport in animals · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Capillaries form dense networks throughout body tissues, linking arteries to veins. They are the site of exchange of oxygen, carbon dioxide, glucose and other substances between the blood and respiring cells.

Model answer (5 marks)

Capillary walls are only one cell thick, consisting of a single layer of endothelial cells. This makes the diffusion pathway very short, allowing oxygen, carbon dioxide, glucose and other substances to move rapidly between the blood and the surrounding tissues.

The lumen of a capillary is extremely narrow, so red blood cells are forced to pass close to the vessel wall. This further reduces the distance that oxygen and other molecules must diffuse from the red blood cells to the body cells.

Capillaries also form extensive, dense networks throughout tissues, providing a very large total surface area for exchange. Together, these structural features make capillaries highly efficient for mass transport between blood and cells.

Examiner tips

  • Mention the single-cell wall thickness first to show understanding of diffusion distance.
  • Explain how the narrow lumen brings red cells close to the wall, reducing diffusion distance.
  • Highlight the large surface area from dense networks as a key point.
  • Use the exact terms: endothelial cells, diffusion pathway, lumen, surface area.

Common mistakes

  • Failing to state that the wall is only one cell thick.
  • Confusing capillary diameter with lumen size or ignoring the effect on diffusion distance.
  • Omitting the importance of the extensive network and surface area.

Mark scheme (5 marks)

  1. Capillary walls are only one cell thick (a single layer of endothelial cells)
  2. This short/thin diffusion pathway means substances can diffuse quickly between blood and cells
  3. Capillaries have a very small lumen (narrow bore), so red blood cells are squeezed close to the wall
  4. This further reduces the diffusion distance for oxygen (and other substances) from red blood cells to body cells
  5. Capillaries form extensive/dense networks, giving a large surface area for exchange

Key terms in this question

capillary

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