The biconcave red blood cells that carry oxygen from the lungs to the tissues, and return carbon dioxide (see also RESPIRATION). They have an excess of membrane, some of which may be lost in various disorders, as a result of which they become progressively more spherical and rigid. Erythrocytes, which have no nuclei, are formed during ERYTHROPOEISIS from ERYTHROBLAST cells in the BONE MARROW, and each mm³ of blood contains 5 million of them. They are by far the largest constituent among the blood cells and they contain large amounts of the oxygen carrier HAEMOGLOBIN. They have a life of about 120 days after which they are absorbed by MACROPHAGES, the blood's scavenging cells. Most components of the erythrocytes, including the red pigment HAEMOGLOBIN, are re-used, though some of the pigment is broken down to the waste product BILIRUBIN.