# Explain how the structure of a vein is adapted to return blood to the heart at low pressure.

> IB DP Biology Standard Level (2023 syllabus) — B3.2 Transport · Explain · 4 marks

## Mark scheme (4 marks)

1. Veins have a wide lumen, which reduces resistance to blood flow / allows large volumes of blood to move with minimal pressure.
2. Veins have thin, less muscular walls / less smooth muscle and elastic tissue than arteries, because they do not need to withstand high pressure.
3. Veins contain semi-lunar / pocket valves that prevent backflow of blood.
4. Skeletal muscle contractions (and breathing movements) compress veins, helping to push blood towards the heart / providing the force needed to move blood at low pressure.

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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