CS - blood vessels

• 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
• (A)the heart pumps the blood out at high pressure so the artery walls are strong and elastic
• (A) the walls are thick compared to the size of the hole down the middle (the lumen)
• (A) they contain thick layers of muscle to make them strong and elastic fibres to allow them to stretch and spring back
• (C) arteries branch into capillaries
• (C) capillaries are really tiny - too small to see
• (C) they carry the blood really close to every cell in the body to exchange substances with them
• (C) they have permeable walls, so substances can diffuse in and out
• (C) they supply food and oxygen, and take away waste like C02
• (C) their walls are usually only one cell thick. this increases the rate of diffusion by decreasing the distance over which it occurs
• (V) capillaries eventually join up to form veins. the blood is at the lower pressure in the veins so the walls don't need to be as thick as artery walls
• (V) they have a bigger lumen than arteries to help the blood flow despite the lower pressure
• (V) they also have valves to help keep the blood flowing in the right direction
• rate of blood flow = volume of blood / number of minutes