Your arteries carry blood from your heart to the rest of your body. Blood travels in the arteries into capillaries. Capillaries are so small that only one cell can fit through at a time. In the capillaries oxygen is taken by the cells and the waste (carbon dioxide) is released into the blood. The blood then enters the veins to get the blood back to the heart.
Your blood is made up of four main parts – red blood cells, white blood cells, platelets and plasma. Red blood cells deliver oxygen from the lungs to the rest of the body and bring carbon dioxide from the body to the lungs. White blood cells fight infection and kill germs. Platelets help to form clots and prevent bleeding. Blood cells travel through the circulatory system in a yellow fluid called plasma.