B3 - Blood Vessels

Blood vessels are designed for their function Arteries - these carry blood away from the heart Capillaries - these are involved in the exchange of materials of the tissues Veins - these carry blood into the heart

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 lumen (the hole in the middle of the artery). They contain thick layers of muscle to make them strong

Capillaries are really small Arteries branch into capillaries Capillaries are really tiny - you can't even see them They carry the blood really close to every cell in the body to exchange substances with them They have permeable walls, so substances can diffuse in and out They supply food and oxygen, and take away wastes like CO2 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 Capillaries eventually join up to form veins The blood is at a low pressure in the veins so the walls don't need to be as thick as artery walls need to be They have a bigger lumen than arteries to help the blood flow despite the lower pressure They also have valves to help keep the blood flowing correctly

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