Overview of iron homeostasis. The central portion of the figure depicts the flow of iron into the body (through the small intestine), to transferrin (Tf), to the major site of utilization (the erythroid bone marrow), to circulating erythrocytes, to tissue macrophages that phagocytose senescent erythrocytes and recycle iron (spleen), to storage in hepatocytes, and back to TF through mobilization of iron stores. Cellular iron transport is described in detail in the text and shown in schematic form on the outside edges of this figure. (A) Nonheme iron transport across an intestinal enterocyte. (B) Erythrophagocytosis and iron recycling in a tissue macrophage. The aqua oval in the cytoplasm represents a storage depot for ferroportin protein within the cell. (C) Hepatocyte iron transport, with arrows indicating that neither import nor export is well understood. (D) Iron uptake through the transferrin cycle in the erythoblast. Illustration by Kenneth Probst.