Erythrocyte (red blood cell)

Shape. The red cell is a biconcave disc about 7.5–8 µm across, about 2–2.5 µm thick at the rim and about 1 µm thick at the centre. This shape gives a large surface area for gas exchange. A flexible skeleton under the membrane, built from spectrin, ankyrin and band 3, lets the cell deform enough to squeeze through capillaries and through the 3-µm slits of the spleen.

Contents. The mature red cell has no nucleus, mitochondria or ribosomes, so it cannot divide or make new proteins. It makes ATP by anaerobic glycolysis. A side branch of glycolysis, the Rapoport–Luebering shunt, produces 2,3-BPG, which regulates how readily haemoglobin releases oxygen. The pentose phosphate pathway, which depends on the enzyme G6PD, supplies NADPH that protects the cell from oxidative damage. Each cell contains about 270–280 million haemoglobin molecules, as well as carbonic anhydrase and band 3, the Cl⁻/HCO₃⁻ exchanger responsible for the chloride shift.

Production (erythropoiesis). Red cells develop in the red marrow in a sequence: the CFU-E progenitor, which depends on erythropoietin, then the proerythroblast, then basophilic, polychromatic and orthochromatic erythroblasts. The orthochromatic erythroblast expels its nucleus to become a reticulocyte, which circulates for 1–2 days before it matures. The whole process takes about a week. It is driven by erythropoietin, made by peritubular fibroblasts of the kidney when tissue oxygen is low, and it needs iron, vitamin B12 and folate.

Removal. Aged red cells become stiffer and expose markers on their surface. Macrophages in the spleen and liver remove them. The globin is broken down to amino acids, and the iron is recycled through transferrin and ferritin. The haem is converted to biliverdin and then to unconjugated bilirubin, which the liver conjugates and excretes in bile.