Explain how the structure of capillaries makes them well adapted for the exchange of substances between the blood and the tissues.

Cambridge International IGCSE Biology (0610) — 9.3 Blood vessels · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Capillaries are only one cell thick, so the diffusion distance for oxygen, carbon dioxide and other substances is very short.
The lumen is narrow, which further reduces the distance between the blood and the surrounding cells.
They are highly branched, giving a very large surface area for exchange.
Blood moves slowly through capillaries, providing more time for diffusion of gases and nutrients.

Examiner tips

Mention the one‑cell wall, narrow lumen, large surface area and slow flow in separate points.
Use the word ‘diffusion’ and ‘exchange’ – these are key terms.
Keep the answer concise – 4 marks, so one sentence per point.

Common mistakes

Confusing capillaries with arteries or veins; they are not muscular. Forgetting to mention the slow blood flow. Using vague terms like ‘thin’ instead of ‘one cell thick’.

Mark scheme (4 marks)

Capillaries have walls that are only one cell thick, which decreases the diffusion distance for substances moving between the blood and tissues.
Capillaries have a narrow lumen / narrow diameter, which also reduces the diffusion distance between the blood and surrounding cells.
Capillaries are highly branched and have a large surface area, increasing the area over which exchange can occur.
Blood flows slowly through capillaries, allowing more time for substances such as oxygen and carbon dioxide to diffuse between the blood and tissues.

Key terms in this question

capillaries · exchange