Explain how the structure of capillaries makes them well suited for the exchange of substances between the blood and the tissues.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Capillaries have walls that are only one cell thick, which makes them very thin. This thinness reduces the diffusion distance, allowing substances to move quickly between the blood and the tissues. Capillaries form a large network and are numerous, giving a large surface area for exchange. Substances move by diffusion down a concentration gradient between the blood and the tissues.
Examiner tips
- Use the word ‘thin’ or ‘one cell thick’ to show understanding of structure.
- Mention both reduced diffusion distance and large surface area.
- Explain that diffusion is driven by a concentration gradient.
- Keep the answer concise and directly linked to the question.”]
Mark scheme (4 marks)
- Capillaries have walls that are only one cell thick / very thin walls
- This reduces the diffusion distance so substances can move quickly between blood and tissues
- Capillaries form a large network/are numerous, giving a large surface area for exchange
- Substances move by diffusion down a concentration gradient between the blood and tissues
Key terms in this question
Related
- All Edexcel GCSE Biology (1BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Gas exchange questions
- During exercise, the breathing rate of a student increases. Explain how this hel…
- Explain how the alveoli are adapted to allow efficient gas exchange in the lungs…
- Explain why a single-celled organism does not need a specialised exchange surfac…
- Explain why a large multicellular animal, such as a fish, requires a specialised…