Explain how the structure of a capillary is adapted for the exchange of substances between the blood and body cells.

AQA A-Level Biology (7402) — 3.3.4 Mass Transport · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Capillaries are the smallest blood vessels and are only one cell thick, giving a very short diffusion distance so that oxygen, carbon dioxide, nutrients and waste products can move quickly between the blood and body cells.
The walls are also very thin and highly permeable, allowing substances to pass through easily.
Capillaries form an extensive network throughout tissues, giving a very large surface area for exchange.
The large surface area increases the rate of diffusion of substances.
Blood flow through capillaries is slow, which gives cells more time to take up oxygen and nutrients and to release waste products.

Examiner tips

  • Mention the one‑cell‑thick wall and short diffusion distance first. Highlight the large surface area and slow blood flow. Use the words ‘diffusion’, ‘permeable’, ‘exchange’ and ‘rate’ to match the mark scheme.
  • common_mistakes
  • :
  • Forgetting to state that the wall is only one cell thick. Not mentioning the slow blood flow. Using vague terms like ‘thin’ without linking to diffusion distance.

Mark scheme (5 marks)

  1. Capillaries have walls that are only one cell thick
  2. This gives a short diffusion distance (so substances can move quickly/easily between blood and cells)
  3. Capillaries have a large surface area (due to forming an extensive network throughout tissues)
  4. This increases the rate of diffusion / exchange of substances
  5. Blood flow through capillaries is slow, giving more time for exchange of substances

Key terms in this question

capillary · exchange of substances

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