Describe how the structure of arteries, veins and capillaries is adapted to their functions in the human circulatory system.

WJEC A-Level Biology (Wales) — 2.1 Classification and biodiversity · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

Arteries have thick, muscular and elastic walls that can withstand and maintain the high pressure of blood pumped from the heart.

Veins have thin walls with less muscle and elastic tissue because the pressure in the venous system is low.

Capillaries are very thin‑walled and have a narrow lumen, which reduces the diffusion distance and allows efficient exchange of gases, nutrients and waste between the blood and tissues.

Examiner tips

  • Use the exact terms "thick muscular and elastic walls" for arteries, "thin walls" for veins, and "thin‑walled, narrow lumen" for capillaries. Explain the functional link: high pressure → thick walls, low pressure → thin walls, diffusion → thin walls. Include the valve function for veins.
  • common_mistakes
  • :
  • Confusing the wall thickness of arteries and veins. Omitting the valve function in veins. Describing capillaries as having a wide lumen instead of a narrow one.

Mark scheme (5 marks)

  1. Arteries have thick muscle and elastic layers / thick walls to withstand and maintain high blood pressure (or to prevent bursting).
  2. Arteries carry blood away from the heart at high pressure.
  3. Veins have thin walls / thin muscle and elastic layer because blood is at low pressure.
  4. Veins contain valves to prevent backflow of blood.
  5. Capillaries have a thin wall / narrow lumen to decrease diffusion distance, allowing exchange of substances between the blood and tissues.

Key terms in this question

vein

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