Biology editorial review is supported by the
named review panel.
Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
The key difference
Arteries carry blood away from the heart under high pressure with thick muscular walls. Veins return blood to the heart under low pressure with thin walls and valves. Capillaries are one-cell-thick vessels where exchange of substances occurs.
The three types of blood vessel are structurally adapted for their specific functions. This is a favourite comparison question: examiners expect you to link each structural feature to its function, not just list differences. Understanding why each vessel is built the way it is will earn you full marks.
Feature
Arteries
Veins
Capillaries
Direction of blood flow
Away from the heart
Towards the heart
Between arteries and veins, through tissues
Blood pressure
High
Low
Low (blood slows for exchange)
Wall thickness
Thick: to withstand high pressure
Thinner than arteries
One cell thick: short diffusion distance
Muscle and elastic tissue
Thick layer of muscle and elastic fibres: recoil maintains pressure
Thin muscle layer: less pressure to withstand
No muscle or elastic tissue
Lumen size
Small (relative to wall thickness)
Large: reduces resistance to flow
Very narrow: only one red blood cell wide
Valves
No valves (except at base of aorta and pulmonary artery)
Valves present: prevent backflow of blood
No valves
Main function
Carry oxygenated blood to organs (except pulmonary artery)
Return deoxygenated blood to heart (except pulmonary vein)
Allow exchange of substances between blood and tissues
Structure-Function Links Examiners Want
It is not enough to state a structural difference: you must explain why:
Arteries have thick muscular walls because blood leaves the heart under high pressure and the walls must withstand this without bursting. The elastic fibres recoil after each heartbeat, helping to smooth the pulsatile flow.
Veins have valves because blood pressure is low by the time it reaches the veins, and without valves, blood would flow backwards under gravity (especially in the legs).
Capillary walls are one cell thick because this provides a short diffusion distance, allowing rapid exchange of oxygen, carbon dioxide, glucose, and urea between the blood and surrounding cells.
The Exception Questions
Examiners often test whether you know the exceptions:
The pulmonary artery carries deoxygenated blood (from heart to lungs): it is still an artery because it carries blood away from the heart
The pulmonary vein carries oxygenated blood (from lungs to heart): it is still a vein because it carries blood towards the heart
The umbilical vein in a fetus also carries oxygenated blood
The rule is: arteries = away from heart, veins = towards heart. Oxygenation is not the defining feature.
Try a comparison question (3 marks)
Describe how the structure of a capillary is adapted for the exchange of substances. [3]
Show the answer and marking guidance
Wall is one cell thick, providing a short diffusion distance for rapid exchange of oxygen, glucose, and carbon dioxide
Lumen is very narrow so red blood cells are squeezed through in single file, slowing blood flow and bringing them close to the capillary wall
Capillaries form extensive networks (capillary beds) providing a large surface area for exchange
Original Cambridge-style practice written for this site.
Common comparison mistakes
When answering a compare question, write matched pairs rather than two separate descriptions.
Saying arteries carry oxygenated blood: the pulmonary artery carries deoxygenated blood; arteries carry blood AWAY from the heart
Stating that veins have thick walls: veins have thinner walls than arteries
Forgetting to explain WHY a feature is present (e.g. Saying capillaries have thin walls without linking this to short diffusion distance)
Confusing lumen size: arteries have a small lumen relative to wall thickness, veins have a large lumen