The three different types of blood vessel
Arteries - These carry the blood away from the heart.
Capillaries - These are involved in the exchange of materials at the tissues.
Veins - These carry the blood to the heart.
Arteries carry blood under pressure
1) The heart pumps the blood out at high pressure so the artery walls are strong and elastic.
2) The walls are thick compared to the size of the hole down the middle (the "lumen")
3) They contain thick layers of muscle to make them strong, and elastic fibres to allow them to stretch and spring back.
Capillaries are really small
1) Arteries branch into capillaries.
2) Capillaries are really tiny - too small to see.
3) They carry the blood really close to every cell in the body to exchange substances with them.
4) They have permeable walls, so substances can diffuse in and out.
5) They supply food and oxygen, and take away waste like carbon dioxide.
6) Their walls are usually only one cell thick. This increases the rate of diffusion by decreasing the distance over which it occurs.
Veins take blood back to the heart
1) Capillaries eventually join up to form veins.
2) The blood is at lower pressure in the veins so the walls don't need to be as thick as artery walls.
3) They have a bigger lumen than arteries to help the blood flow despite the lower pressure.
4) They also have valves to help keep the blood flowing in the right direction.