On the school field, we recreated the circulation of blood around the body. One of the students represented the heart, two others the lungs, two the kidneys and others the limbs etc. Most of the children were acting as red blood cells and one or two, as white blood cells. The blood cells then had to run from the heart, to the lungs, back to the heart, then to the kidneys to clean the blood, then to one of the limbs and then back to the heart, all in 20 seconds – the average time that blood completes one circulatory circuit.