False colour SEM showing normal red blood cells. Blood is composed of 50% water and 45% red blood cells or erythrocytes which carry oxygen to the body's tissues. Oxygen is bound to haemoglobin, an iron-containing protein. The combination of haemoglobin and oxygen is responsible for the red colour of blood. About 150 red blood cells placed side by side would fit across the head of a pin. Photographed using a Philips 501 scanning electron microscope.Source: Electron Microscopy Unit, Royal Free Hospital School of Medicine, London.