Explain how the structure of a vein makes it well suited to returning blood to the heart.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 1.3 Mass transport in animals · Explain · 5 marks · View as Markdown

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

Blood is transported around the body in three types of blood vessel: arteries, capillaries and veins. Veins carry blood from the organs back towards the heart, often against the force of gravity.

Model answer (5 marks)

Veins have a wide lumen, which reduces resistance and allows blood to flow easily at the low pressures that exist in the venous system.

The walls of veins are thin and contain little smooth muscle or elastic tissue, so they do not need to withstand high pressures.

Veins also contain valves that prevent back‑flow, ensuring that blood moves only towards the heart even when it must travel against gravity.

Examiner tips

Mention the wide lumen first, then the thin walls, then the valves; this order matches the mark scheme.
Use terms like "low pressure" and "prevent back‑flow" to show understanding of function.
Keep the answer concise – 5 marks can be earned with three short points.

Common mistakes

Confusing arteries with veins – students sometimes describe arterial features such as thick muscular walls.
Failing to mention the valves or the low pressure aspect – both are required for full marks.
Writing in a narrative style that repeats the question rather than directly addressing the points.

Mark scheme (5 marks)

Veins have a wide lumen
Wide lumen reduces resistance / allows blood to flow easily / allows blood to flow at low pressure
Veins have valves
Valves prevent the backflow of blood / ensure blood flows in one direction (towards the heart)
Veins have thin walls / less muscle and elastic tissue than arteries, which is sufficient because blood is at low pressure

Key terms in this question

vein