Arteries take blood away from the heart. The walls of an artery are made up of thick muscular walls and elastic fibres. Veins carry blood towards the heart and also have valves. Capillaries link arteries and veins, and have a one cell thick wall. Blood is made up of four main things: plasma, the liquid part of the blood; Red Blood Cells to carry oxygen; White Blood cells to protect the body from disease and platelets to help blood clot.