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Red blood cells. Coloured scanning electron micrograph (SEM) of a section through a freeze- fractured blood vessel, showing human red blood cells (erythrocytes, red). These biconcave, disc-shaped cells transport oxygen from the lungs to body cells. They also remove carbon dioxide to the lungs for exhalation. Their red colour comes from the oxygen-carrying protein haemoglobin. Red blood cells are the most abundant cell in the blood. The endothelial lining of the blood vessel (pink) is also seen. Magnification: x200 at 6x7cm size. (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE GSCHMEISSNER)