these are involved in the exchange of materials at the tissues
veins
these carry blood to the heart
arteries carry blood under pressure
the heart pumps the blood out at a high pressure so the artery walls are strong and elastic. the walls are thick compared to the lumen. they contain thick layers of muscle to make them strong and spring back
capillaries are really small
arteries branch into capillaries. they carry blood really close to every cell to exchange substances with them. they supply food and oxygen and take away wastes like CO2. the 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
capillaries eventually join up to form veins the blood is carried at a lower pressure as they done need to be as thick. they have bigger lumen. they also have valves to help keep the blood flowing in the rightdirection
