blood vessels

• arteries - carry blood away from heart
• capillaries - these are involved in the exchange of materials at the tissue
• veins - carry blood into heart
• the heart pumps blood out at a high pressure so artery walls are strong and elastic. the walls are thick compared to the size of the lumen (the inside)
• arteries contain a thick layer of muscle to make them strong and elastic fibres to allow them to stretch
• arteries branch into capillaries, which are really tiny blood vessels
• capillaries carry the blood really close to the cells of the body so that substances can be exchanged and have permeable walls for diffusion
• capillaries supply food and oxygen and take away waste like CO2
• capillaries walls are usually one cell thick which increases rate of diffusion by decreasing the distance
• capillaries eventually join up to form veins that take blood back to heart
• vein walls dont need to be as thick as the blood has less pressure so the walls are thinner
• veins have a bigger lumen then arteries to help blood flow despite less pressure
• veins also have valves to keep blood flowing in the right direction
• valves prevent backflow of blood when muscles contract, they close off the lumen preventing blood from going backwards