Skip to content

2.11 Blood vessels

How the structures of arteries, veins and capillaries suit their jobs: thick elastic walls for high pressure, valves and wide lumens for low pressure, and walls one cell thick for exchange, plus how to calculate the rate of blood flow.

AQA Edexcel OCR 2 min read

Three types of blood vessel

Key terms

  • Artery: a blood vessel that carries blood away from the heart, at high pressure.
  • Vein: a blood vessel that carries blood back to the heart, at low pressure.
  • Capillary: a tiny blood vessel where substances are exchanged with body cells.
  • Lumen: the space inside a blood vessel, where the blood flows.

Blood leaves the heart in arteries. These branch into capillaries, which run through every tissue. Capillaries join up to form veins, which take the blood back to the heart.

Cross-sections of an artery with a thick muscular and elastic wall and narrow lumen, a vein with a thin wall and large lumen, and a capillary with a wall one cell thick
Cross-sections of an artery with a thick muscular and elastic wall and narrow lumen, a vein with a thin wall and large lumen, and a capillary with a wall one cell thick
How structure suits function
VesselStructureWhy
ArteryThick walls of muscle and elastic fibres; narrow lumenWithstands the high pressure from the heart; elastic fibres stretch and recoil with each beat
VeinThin walls; large lumen; valvesBlood is at low pressure; the large lumen lets it flow easily; valves stop it flowing backwards
CapillaryWall one cell thick; very narrow; huge numbersShort diffusion distance for exchange; reaches every cell

Valves in veins

The pressure in veins is low, so blood could flow backwards, especially in the legs. Valves stop this. Body muscles squeeze the veins as they contract, pushing the blood towards the heart.

A valve in a vein opens when blood flows towards the heart and closes when blood is pushed backwards
A valve in a vein opens when blood flows towards the heart and closes when blood is pushed backwards

Exchange in capillaries

Capillary walls are one cell thick and are often leaky. Oxygen, glucose and other substances diffuse out of the blood into the body cells, and carbon dioxide and urea diffuse from the cells into the blood. Capillaries are so narrow that red blood cells pass in single file, close to the wall.

Rate of blood flow

rate of blood flow = volume of blood / time

Example: 2400 cm3 of blood flows through an artery in 4 minutes. Rate = 2400 รท 4 = 600 cm3 per minute. Blood flow is fastest in arteries and slowest in capillaries, which gives more time for exchange.

Test yourself on this lesson

6 exam questions · 12 marks · with mark schemes and model answers

Start questions

Found a mistake?