Ben Silburn presents the series that explores the ends of the universe, the centre of the Earth and the depths of the human body. This week he examines a red blood cell as it travels from the heart, through the lungs and round the body inside a top athlete running in an international event. The cell has to deliver oxygen and nutrients to the muscles but is diverted when a spike from the shoe of a fellow competitor pierces the skin. Producer Paul Arnold. E-MAIL: scirad@bbc.co.uk