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Arteries, veins and capillaries

Structure and functions in living organisms · The heart and circulation · note 4 of 5

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Arteries, veins and capillariesSpec 2.68

In short

Arteries, veins and capillaries are the three types of blood vessel. Arteries carry blood away from the heart at high pressure, so they have thick, muscular walls with elastic fibres. Veins carry blood back to the heart at low pressure, with a wide lumen and valves to stop backflow. Capillaries have walls one cell thick for exchanging substances with cells.

There are three types of blood vessel. Each has a structure that suits its function.

Arteries, veins and capillaries
ArteryVeinCapillary
Direction of bloodAway from the heartTowards the heartBetween arteries and veins, through the tissues
Blood pressureHighLowFalls as blood passes through
WallThick, muscular, with elastic fibresThinner wallOne cell thick
LumenNarrowWideVery narrow
ValvesNoYesNo
FunctionCarry blood at high pressure to the organsCarry blood back to the heartExchange substances with cells
  • Arteries have thick, muscular walls with elastic fibres. The wall can stand the high pressure, and it stretches and springs back with each heartbeat.
  • Veins carry blood at low pressure. They have a wide lumen, which lets blood flow easily, and valves to stop the blood flowing backwards.
  • Capillaries have walls only one cell thick, so there is a short diffusion distance. They are very narrow and form a huge network, so they reach every cell and give a large surface area for exchange.
Cross-sections of an artery with a thick wall of muscle and elastic fibres and a narrow lumen, a vein with a thin wall and a wide lumen, and a capillary with a wall one cell thick and a very narrow lumen, plus a vein cut lengthways showing a valve that lets blood flow only towards the heart. (opens full size in a new tab)
Arteries have thick walls, veins have a wide lumen and valves, capillaries have walls one cell thick. (The capillary is drawn much larger than its real size.)
Exam tip:

Do not say that arteries always carry oxygenated blood and veins always carry deoxygenated blood. The pulmonary artery and pulmonary vein are the exceptions.

Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026

Frequently asked questions

Why is the left ventricle thicker than the right?

The left ventricle has a thicker, more muscular wall than the right because it pumps blood all the way around the body through the aorta, which needs a higher pressure. The right ventricle pumps deoxygenated blood only to the nearby lungs through the pulmonary artery, so it needs less pressure and a thinner wall.

How does the heart pump blood?

The heart pumps blood by contracting its muscular chambers. Deoxygenated blood enters the right atrium from the vena cava, passes to the right ventricle and is pumped to the lungs. Oxygenated blood returns to the left atrium in the pulmonary vein, passes to the left ventricle and is pumped to the body through the aorta. Valves prevent backflow.

Why do arteries have thick elastic walls?

Arteries have thick, muscular walls with elastic fibres because they carry blood away from the heart at high pressure. The thick wall can withstand the high pressure, and the elastic fibres let it stretch and spring back with each heartbeat. Arteries have a narrow lumen and no valves.

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