The mammalian iron cycle.

Dietary iron enters the cycle via the absorptive enterocyte. Iron absorbed from the gut lumen is transferred to plasma, bound to circulating apo-transferrin (Tf) and utilized by cells expressing transferrin receptors, mainly red cell precursors. Circulating red cells have a lifetime of approximately 120 days after which they are ingested by tissue macrophages. Heme oxygenase expressed in the macrophage liberates iron from heme, and the iron is recycled to plasma where it again binds to circulating apo-transferrin. Some fraction of dietary and macrophage recycled iron is deposited for storage in hepatocytes as ferritin-bound iron. Hepatocyte iron can be mobilized and returned to the circulation. CP, ceruloplasmin; Hp, hephaestin. For other symbols see Figure 2.