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 · View as Markdown

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 of blood to the heart by having a wide lumen, which reduces resistance and allows large volumes of blood to flow with minimal pressure. Their walls are thin and contain less smooth muscle and elastic tissue than arteries, so they do not need to withstand high pressure. Semi‑lunar valves are present in veins to prevent backflow. Finally, skeletal‑muscle contractions and breathing movements compress the veins, providing the force that pushes blood toward the heart at low pressure.

Examiner tips

  • Use the term ‘wide lumen’ and ‘low resistance’ to show understanding of flow mechanics.
  • Mention ‘thin walls, less smooth muscle’ to explain structural adaptation.
  • Include ‘semi‑lunar valves’ and ‘skeletal‑muscle pump’ to cover all key points.
  • Keep each point concise and directly linked to the question.”]

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.

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