Red blood cells. Animated 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.