Explain how the structure of arteries, veins and capillaries is related to their functions in the human circulatory system.
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, preventing the vessels from bursting. Veins have thinner walls with less muscle and elastic tissue because the blood pressure is lower; they also contain valves that prevent back‑flow of blood. Capillaries have very thin walls and a very small diameter, which reduces the diffusion distance and allows rapid exchange of oxygen, nutrients and waste between the blood and tissues. Their extensive network gives a large surface area so that every cell is close to a capillary, and the slow flow of blood gives time for this exchange to occur.
Examiner tips
- Mention wall thickness and composition for arteries and veins; include valves in veins; emphasise thin walls and large surface area for capillaries; link structure to function (pressure, flow, exchange).
Common mistakes
- Confusing the wall thickness of arteries and veins; forgetting valves in veins; not linking capillary structure to diffusion distance or surface area.
Mark scheme (5 marks)
- Arteries have thick muscle and elastic layers / thick walls to withstand or maintain high blood pressure (accept: to prevent bursting)
- Veins have thin muscle and elastic layers / thin walls because blood is at low pressure
- Veins contain valves to prevent backflow of blood
- Capillaries have a thin wall / thin diameter to decrease the diffusion distance, allowing exchange of substances between the blood and tissues
- Capillaries have a large surface area / extensive network so that every cell in the body is close to a capillary (accept: slower blood flow allows time for exchange)
Key terms in this question
Related
- All Eduqas A-Level Biology revision notes →
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- Decode the mark scheme abbreviations →
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