Blood Vessels

• Arteries carry blood away from the heart
• Capillaries are involved in the exchange of materials at the tissues
• Veins carry blood to the heart
• Arteries carry blood under pressure

  • Artery walls are strong and elastic
  • The walls are thick compared to the lumen (the hole in the middle)
  • Thick layers of muscle to make them strong and elastic fibres so they can stretch and spring back
• Capillaries are really small

  • Arteries branch into capillaries
  • Capillaries are too small to see
  • They carry blood close to every cell in the body to exchange substances
  • They have permeable walls to allow for diffusion
  • They supply food and oxygen and take away waste (carbon dioxide)
  • Their walls are one cell thick to increase the rate of diffusion by decreasing the distance over which it occurs.
• Veins take blood back to the heart

  • Capillaries join up to form veins
  • The blood is at a lower pressure so the walls do not need to be as thick
  • They have a bigger lumen to help blood flow
  • They also have valves to keep blood flowing in the right direction
• Rate of blood flow = volume of blood / number of minutes