Arteries, Veins and Capillaries: Structure and Function
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Arteries carry blood away from the heart at high pressure, with thick muscular walls and narrow lumens. Veins return blood at low pressure, with thinner walls, wider lumens and valves. Capillaries are tiny, one-cell-thick vessels where exchange occurs between blood and tissues.
This comparison appears almost every exam series. The table format is the most efficient way to answer, but always link each structural feature to the specific pressure or exchange function it supports.
Arteries
Arteries carry blood away from the heart. The heart pumps blood at high pressure, so arteries have thick muscular walls with elastic fibres that stretch and recoil with each heartbeat, smoothing the pulsatile flow. The lumen is relatively narrow compared to the wall thickness, maintaining high pressure.
The largest artery is the aorta, which receives oxygenated blood from the left ventricle. Arteries branch into smaller arterioles, which further branch into capillary networks.
Veins
Veins carry blood toward the heart at low pressure. Their walls are thinner with less muscle and elastic tissue, and the lumen is wider, reducing resistance to flow. Since blood pressure in veins is too low to prevent backflow, veins contain valves that ensure blood moves only toward the heart.
Skeletal muscles surrounding veins contract during movement and squeeze blood along: this is the skeletal muscle pump. Prolonged standing without movement can cause blood to pool in leg veins.
Capillaries and the comparison table
Capillaries are the smallest blood vessels, connecting arterioles to venules. Their walls are one cell thick, providing a very short diffusion distance for exchange of oxygen, carbon dioxide, glucose, urea and other substances between blood and surrounding tissues. Capillaries form dense networks (capillary beds) that pass close to every cell.
Feature
Artery
Vein
Capillary
Direction
Away from heart
Toward heart
Between arterioles and venules
Wall thickness
Thick (muscle + elastic)
Thinner
One cell thick
Lumen size
Narrow
Wide
Very narrow (one RBC wide)
Valves
No (except semilunar)
Yes
No
Blood pressure
High
Low
Decreasing
Try a focused question (2 marks)
Explain why the wall of a capillary is one cell thick.
Show the answer and marking guidance
A one-cell-thick wall provides a short diffusion distance.
This allows rapid exchange of substances such as oxygen and glucose between blood and tissues.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.