The other cells in the blood, the white blood cells, are actually immune cells.
Erythrocytes deliver oxygen via hemoglobin, a complex molecule containing heme (iron) groups that temporarily link to oxygen molecules in the lungs or gills and release them throughout the body. Hemoglobin also carries some of the waste product carbon dioxide back from the tissues. (Less than 2% of the total oxygen, and most of the carbon dioxide are also held in solution in the blood plasma.) Red blood cells consist of almost 90% hemoglobin; the heme is what gives blood its red color. A related compound, myoglobin, acts to store oxygen in muscle cells.
Erythrocytes in mammals are anucleate when mature, meaning that they lose their cell nucleus and thus their DNA. (Amphibian and bird erythrocytes have nuclei.) Erythrocytes also lose their mitochondria and produce energy from glucose, via glycolysis followed by lactic acid fermentation.
Human erythrocytes have a flattened ovate shape, depressed in the center. This shape is optimized for the exchange of oxygen with the surroundings. The cells are flexible so as to fit through tiny capillaries, where they release their oxygen load. The diameter of a typical erythrocyte is 6-8 µm. The blood types of humans are due to variations in surface glycoproteins of erythrocytes.
Adult humans have roughly 2-3 × 1013 red blood cells at any given time (women have about 4-5 million erythrocytes per cubic millimeter of blood and men about 5-6 million). Erythrocytes are continuously being produced in the red bone marrow of large bones. (In the embryo, the liver is the main site of red blood cell production.) The production can be stimulated by the hormone erythropoietin. Erythrocytes develop in about 7 days and live a total of about 120 days. The aging cells swell up to a sphere-like shape and are engulfed by phagocytes, destroyed and their materials are released into the blood. The hemoglobin is eventually excreted as bilirubin.
The spleen acts as a reservoir of red blood cells, but this effect is somewhat limited in humans. However, in some other mammals such as dogs and horses, the spleen sequesters large numbers of red blood cells that are dumped into the blood during times of exertion stress. Some athletes have tried doping their blood in the same way with their own packed cells, but this practice endangers the human cardiovascular system because it is not equipped to deal with blood of the resulting higher viscosity.
Diseases involving the red blood cells include: