Erythroid precursors mature in the bone marrow in physical contact with a central macrophage (A). After completing hemoglobinization, erythroid precursors extrude their nucleus and enter the circulation (B). Senescent erythrocytes are phagocytosed by tissue macrophages, where the iron is recycled (C). Iron is returned to the erythron via diferric transferrin (D). (Figure courtesy of the author.)