Explain how the structure of a vein is adapted to allow blood to flow back to the heart.

WJEC A-Level Biology (Wales) — 2.4 Adaptations for nutrition · Explain · 5 marks · View as Markdown

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

Veins carry deoxygenated blood from the body's tissues back to the heart. Unlike arteries, veins carry blood at low pressure.

Model answer (5 marks)

Veins have thin walls and a thin smooth‑muscle layer because the blood pressure is low, so a thick wall would be unnecessary and would restrict flow.

They contain valves in the walls. The valves open when blood moves toward the heart and close when the pressure falls, preventing backflow.

The lumen of a vein is large, giving a wide central channel. A large lumen reduces resistance, allowing blood to move easily at the low pressure present in veins.

These features together enable blood to return to the heart against gravity and at low pressure.

Examiner tips

  • Use the word ‘adapted’ and link structure to function. Include all five points in a logical order. Show how each feature (thin wall, valves, large lumen) helps low‑pressure flow.
  • common_mistakes
  • :
  • Forgetting to mention the valves or their function. Describing arteries instead of veins. Using vague terms like ‘big’ instead of ‘large lumen’.

Mark scheme (5 marks)

  1. Veins have thin walls / thin muscle and elastic layer because blood is at low pressure
  2. Veins contain valves
  3. Valves prevent backflow of blood (so blood moves in one direction towards the heart)
  4. Veins have a large lumen (wide central channel)
  5. The large lumen allows blood to flow easily / at low pressure back to the heart

Key terms in this question

vein

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