The most numerous type of blood cell, which contains the red pigment haemoglobin and is responsible for oxygen transport and for transport of some carbon dioxide (as a carbamino group). When mature, mammalian erythrocytes are disc-shaped and lack a nucleus and mitochondria; those of other vertebrates are oval and nucleated. Human erythrocytes are 7–8 µm in diameter and number some 5 to 6 million per cubic millimetre of blood. Each is capable of carrying about 1 billion molecules of oxygen. They survive for about four months and are then destroyed in the spleen and liver. See also erythropoiesis. Compare leucocyte.