A schematic representation of the human erythron (erythroid marrow plus circulating RBCs), the kidney oxygen-sensing apparatus, and the reticuloendothelial system that phagocytizes senescent RBCs and returns iron to the erythroid marrow. Erythroid hypoproliferation, ineffective erythropoiesis, and peripheral hemolysis are the 3 major categories of a functional classification of anemia. Professional illustration by Somersault18:24.