The arteries carry blood away from the heart, they are made up of three layers of thick muscles and elastic fibers. The muscle contracts to help push blood along the artery, while the elastic fibers allow the artery to stretch when subjected to the high pressure generated by the heart when it beats. Veins carry blood towards the heart and are also made up of three layers of muscular and elastic fibres. Veins have only thin muscles since the blood they carry is under lower pressure than the blood in the arteries. When the body's muscles surrounding the veins move, they compress the vein and push the blood along. Most veins have valves that prevent blood from flowing backwards. Capillaries are the connection between arteries and veins, the capillary wall is only one cell thick to allow for easy exchange of substances between the blood and body cells. Blood is the fluid that transports many substances in the body; oxygen, carbon dioxide, hormones, waste, nutrients. Blood is made up of four main components: plasma, erythrocytes (red blood cells), leukocytes (white blood cells) and thrombocytes (platelets). Platelets and red and white blood cells are produced in the bone marrow. There are four main blood groups: A, B, O and AB. Red blood cells sometimes contain the RhD antigen, if the blood contains this antigen the blood group is RhD positive, if not the blood group is RhD