Red blood cells. False-colour scanning electron micrograph of a group of red blood cells (erythrocytes). They are travelling through an arteriole which is a small branch of an artery. During its short life (about four months) a red blood cell covers about 15km every day for a total of 1500km. In a litre of blood there are normally 5000 billion red blood cells. They transport oxygen from the lungs to the tissues where it is necessary for all metabolic processes. The carbon dioxide produced in these processes is returned to the lungs where it is exhaled. Magnification: x 1740 at 6x7cm size.