Describe how the structure of arteries, veins and capillaries is adapted 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 a thick, muscular and elastic wall, allowing them to withstand and maintain the high pressure of blood pumped from the heart.
Veins have a thinner wall with less muscle and elastic tissue, suitable for the low pressure of blood returning to the heart.
Veins also contain valves that prevent the back‑flow of blood.
Capillaries have a very thin wall and a narrow diameter, which reduces the diffusion distance for oxygen, nutrients and waste products.
Because capillaries form a large surface area and blood moves slowly through them, there is sufficient time for exchange between the blood and surrounding tissues.
Veins have a thinner wall with less muscle and elastic tissue, suitable for the low pressure of blood returning to the heart.
Veins also contain valves that prevent the back‑flow of blood.
Capillaries have a very thin wall and a narrow diameter, which reduces the diffusion distance for oxygen, nutrients and waste products.
Because capillaries form a large surface area and blood moves slowly through them, there is sufficient time for exchange between the blood and surrounding tissues.
Examiner tips
- Use the exact terms: thick muscular/elastic wall for arteries, thin wall for veins and capillaries, valves in veins, large surface area and slow flow in capillaries.
- Show the link between structure and function – e.g. high pressure → thick wall.
- Mention the purpose of valves explicitly.
Common mistakes
- Confusing the wall thickness of arteries and veins; writing that veins are thick.
- Omitting the valves in veins or the large surface area of capillaries.
- Using vague language such as "big" or "small" instead of "thick" or "thin".
Mark scheme (5 marks)
- Arteries have a thick muscle and elastic layer / thick wall to withstand or maintain high blood pressure (preventing bursting).
- Veins have a thin muscle and elastic layer / thin wall because blood is at low pressure.
- Veins contain valves to prevent backflow of blood.
- Capillaries have a thin wall / narrow diameter to decrease diffusion distance, allowing exchange of substances between the blood and tissues.
- Capillaries have a large surface area and slow blood flow to allow time for exchange to take place.
Key terms in this question
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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