Explain how the structure of a capillary is adapted to allow the exchange of substances between the blood and the body's tissues.

WJEC A-Level Biology (Wales) — 2.4 Adaptations for nutrition · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Capillary walls are only one cell thick, which makes them very thin.
This reduces the diffusion distance, allowing substances to move quickly between the blood and tissues.
Capillaries have a very large total surface area.
The large surface area permits many molecules to diffuse across at once, increasing the rate of exchange.
Blood flow through capillaries is slow, giving time for the exchange to take place.

Examiner tips

  • Use the exact terms: 'one cell thick', 'large surface area', 'slow blood flow'.
  • Show the causal link: thin walls → short diffusion distance → rapid exchange.
  • Mention all three adaptations to hit all 5 marks.

Common mistakes

  • Confusing capillaries with arteries or veins; not noting the thinness of the wall.
  • Omitting the slow blood flow point or describing it incorrectly.
  • Using vague phrases like 'thin' without specifying 'one cell thick' or not linking to diffusion distance.

Mark scheme (5 marks)

  1. Capillary walls are only one cell thick / very thin walls
  2. This decreases the diffusion distance (so substances can move quickly between blood and tissues)
  3. Capillaries have a large (total) surface area
  4. This allows more molecules to diffuse across at once / increases rate of exchange
  5. Blood flow through capillaries is slow, allowing more time for exchange to take place

Key terms in this question

capillary

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