Circulatory system - bood vessels

• Arteries carry blood under pressure

  • the heart pumps the blood out at high pressure so the artery walls are strong and elastic
  • the walls are thick compared to the size of the hole down the middle (lumen)
  • contain thick layers of muscles to make them strong, and elastic fibres to allow them to stretch and spring back
• Capillaries are really small

  • arteries branch into capillaries
  • really tiny - too small to see
  • carry the blood close to every cell in the body to exchange substances with them
  • they have permeable walls, so substances can diffuse in and out
  • supply food and oxygen and take away waste
  • walls are usually 1 cell thick, increases the rate of diffusion by decreasing the distance over which it occurs
• Veins take blood back to the heart

  • capillaries eventually join up to form veins, the blood is at lower pressure in the veins so the walls don't need to be as thick as artery walls
  • they have a bigger lumen than arteries to help the blood flow despite the lower pressure
  • also have valves to help keep the blood flowing in the right direction