Iron sensing and the homeostatic regulation of iron absorption, release from stores, and plasma iron content. Iron flows and signals are denoted by red arrows; the putative iron sensor complex and its signaling pathways by green symbols and arrows; and the resulting hepcidin synthesis and secretion by a blue arrow. Importantly, noniron signals, including inflammation (interleukin-6 [IL-6]), and anemia or hypoxia, also regulate hepcidin synthesis but for simplicity are not shown in this diagram.