• The body contains several different types of blood vessel:
  • Arteries: transport blood away from the heart (usually at high pressure)
  • Veins: transport blood to the heart (usually at low pressure)
  • Arterioles: arteries branch into narrower blood vessels called arterioles which transport blood into capillaries
• The walls of each type of blood vessel have a structure that relates to the function of the vessel
• Blood flows through the lumen of a blood vessel; the size of the lumen varies depending on the type of blood vessel (with arteries having a narrow lumen, and the veins a wider one)

The blood vessels form a continuous network; the structure of each allows it to carry out its function

How structure relates to function

• Arteries must be able to withstand high pressures generated by the contracting heart, and maintain these pressures when the heart is relaxed
  • The wall of the artery is relatively thick with layers of collagen, smooth muscle and elastic fibres
  • The elastic fibres allow the artery wall to expand around blood surging through at high pressure when the heart contracts, these fibres then recoil when the heart relaxes – this alongside a narrow lumen maintains high blood pressure
• In contrast, veins receive blood that has passed through capillary networks; blood is at very low pressure and must be returned to the heart
  • The wall of the vein is relatively thin with thinner layers of collagen, smooth muscle and elastic fibres
  • The lumen of the vein is much larger than that of an artery
  • Veins contain valves that prevent the backflow of blood, helping return blood to the heart
• Arterioles can contract and partially cut off blood flow to specific organs
  • Eg. During exercise blood flow to the stomach and intestine is reduced which allows for more blood to reach the muscles
  • Unlike arteries, arterioles have a lower proportion of elastic fibres and a large number of muscle cells
  • The presence of muscle cells allows them to contract and close their lumen to stop blood flow