Red blood cells. Coloured scanning electron micrograph (SEM) of red blood cells (erythrocytes). Red blood cells are biconcave, disc-shaped cells that transport oxygen from the lungs to body cells. They circulate around the body in the blood and 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. They have no nucleus and are about 7 micrometres in diameter. Magnification: x3150 when printed at 10 centimetres tall.