Erythrocyte

An erythrocyte, or red blood cell, is a type of blood cell specialized for gas transport. In humans and other mammals, mature erythrocytes are small, biconcave disks about 6–8 μm in diameter, and are anucleate and devoid of mitochondria. This design provides a large surface-area-to-volume ratio and maximizes flexibility to traverse capillaries.

An erythrocyte's principal function is oxygen transport from the lungs to tissues and the removal of carbon dioxide from tissue metabolism back to the lungs. This capability is enabled by hemoglobin, a tetrameric protein of four globin chains with four heme groups that bind O2 and CO2. Each red cell contains about 270 million hemoglobin molecules, enabling rapid loading and unloading of gases according to partial pressures. Hemoglobin's affinity for oxygen is modulated by factors such as pH, carbon dioxide, temperature, and 2,3-bisphosphoglycerate.

Development occurs by erythropoiesis in the bone marrow, under the regulation of erythropoietin (EPO) produced by the kidneys in response to hypoxia. The maturation process from progenitors to reticulocytes and then erythrocytes takes about a week; circulating erythrocytes have a lifespan of roughly 120 days in humans. Senescent cells are cleared primarily by macrophages in the spleen and liver, and the iron from heme is recycled while bilirubin is excreted in bile.

Metabolically, erythrocytes rely on glycolysis and the pentose phosphate pathway, since they lack mitochondria. They also maintain structural integrity via a cytoskeletal network involving spectrin and ankyrin and maintain gas transport efficiency through regulated hemoglobin function. Abnormalities in erythrocyte number, shape, or hemoglobin production underlie conditions such as anemia, polycythemia, sickle cell disease, and various hemolytic or membrane disorders.