Explain how the structure of a vein is adapted to facilitate the return of blood to the heart.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Veins are adapted for low‑pressure return flow by
1. having a wide lumen, which reduces resistance and allows a large volume of blood to move at low pressure;
2. possessing thin walls with little smooth muscle or elastic tissue, because the low pressure does not require strong walls;
3. containing valves (pocket or semilunar) that prevent backflow and ensure one‑directional flow toward the heart;
4. being compressed by surrounding skeletal muscle during contraction, which pushes blood toward the heart and the valves keep the flow unidirectional.
1. having a wide lumen, which reduces resistance and allows a large volume of blood to move at low pressure;
2. possessing thin walls with little smooth muscle or elastic tissue, because the low pressure does not require strong walls;
3. containing valves (pocket or semilunar) that prevent backflow and ensure one‑directional flow toward the heart;
4. being compressed by surrounding skeletal muscle during contraction, which pushes blood toward the heart and the valves keep the flow unidirectional.
Examiner tips
- Use the word ‘adapted’ and link structure to function; mention low pressure and one‑directional flow; include all four points in a concise paragraph.
- Show the logical sequence: wide lumen → low resistance, thin walls → low pressure, valves → unidirectional, muscle compression → active return.
Common mistakes
- Omitting the role of valves or muscle contraction; confusing veins with arteries; using vague terms like ‘big’ instead of ‘wide lumen’ or ‘thin walls’.
Mark scheme (4 marks)
- Veins have a wide lumen, which reduces resistance to blood flow / allows a large volume of blood to move at low pressure.
- Veins have thin walls / relatively little smooth muscle or elastic tissue, because blood pressure is low / they do not need to withstand high pressure.
- Veins contain valves (pocket/semilunar valves) that prevent the backflow of blood / ensure one-directional flow towards the heart.
- Surrounding skeletal muscle contractions compress veins / exert pressure on vein walls, pushing blood towards the heart; valves ensure this movement is unidirectional.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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