Illustration of red blood cells flowing inside an artery or vein. Red blood cells are biconcave, disc-shaped cells that transport oxygen from the lungs to body cells. They circulate in the blood and also export carbon dioxide to the lungs for exhalation. Their red colour is due to the oxygen-carrying protein haemoglobin. Red blood cells, the most abundant cell in the blood, have no nucleus and are about 7 micrometres across