Circulatory system- blood vessels

• There are three different types of blood vessels:

  Arteries

  Capillaries

  Veins
• Arteries carry the blood away from the heart
• Capillaries are involved in the exchange of materials at the tissues
• Veins carry the blood to the heart
• Arteries carry blood under pressure
• Heart pumps blood out at a high pressure so the artery walls are strong and elastic
• Artery walls are thick compared to the size of the hole down the middle the lumen and contain thick layer of muscle to make them strong and elastic fibres allow them to stretch and spring back
• Arteries branch into capillaries and they are really tiny
• Capillaries are very narrow so the fit between cells and means they can carry the blood really close to every cell in the body to exchange substance with them
• Capillaries have permeable walls so substance can diffuse in and out
• Capillaries supply food and oxygen and takes away waste like carbon dioxide
• Capillary walls are only one cell thick which increases the rate of diffusion as the diffusion distance is small
• Capillaries join up to form veins
• The blood is at a lower pressure in the veins so the walls don't need to be as thick as artery walls
• Veins have bigger lumen than arteries to help the blood flow despite the lower pressure and they also have valves to help keep the blood flowing in the right direction