Blood Vessels: Structure & Function (Edexcel IGCSE Biology)

• There are three main types of blood vessel:
  • Arteries
  • Veins
  • Capillaries
• Smaller vessels that branch off from arteries are called  arterioles (small arteries) and those that branch into veins are called venules (small veins)
• Each vessel has a particular function and is specifically adapted to carry out that function efficiently

Arteries

• Key features:
  • Carry blood at high pressure away from the heart
  • Carry oxygenated blood (except the pulmonary artery)
  • Have thick muscular walls containing elastic fibres
  • Have a narrow lumen
  • Blood flows through at a fast speed
• The structure of an artery is adapted to its function in the following ways
  • Thick muscular walls containing elastic fibres withstand the high pressure of blood and maintain the blood pressure as it recoils after the blood has passed through
  • A narrow lumen also helps to maintain high pressure

Veins

• Key features:
  • Carry blood at low pressure towards the heart
  • Carry deoxygenated blood (other than the pulmonary vein)
  • Have thin walls
  • Have a large lumen
  • Contain valves
  • Blood flows through at a slow speed
• The structure of a vein is adapted to its function in the following ways:
  • A large lumen reduces resistance to blood flow under low pressure
  • Valves prevent the backflow of blood as it is under low pressure

Comparing the structure of arteries and veins

Capillaries

• Key features:
  • Carry blood at low pressure within tissues
  • Carry both oxygenated and deoxygenated blood
  • Have walls that are one cell thick
  • Have ‘leaky’ walls
  • Speed of blood flow is slow
• The structure of a capillary is adapted to its function in the following ways:
  • Capillaries have walls that are one cell thick (short diffusion distance) so substances can easily diffuse in and out of them
  • The ‘leaky’ walls allow blood plasma to leak out and form tissue fluid surrounding cells

Structure of a capillary