iron metabolism, a series of processes involved in the entry of iron into the body and its absorption, transport, storage, use in the formation of hemoglobin and other iron compounds, and eventual excretion. Iron normally enters through the intestinal mucosa and is oxidized from ferrous to ferric iron in the process. The rate at which iron enters is modulated by this absorption mechanism. When iron stores are high, iron no longer passes through but is trapped by the mucosal cells of the intestine to be eliminated. Once in the blood, iron cycles between the plasma and the reticuloendothelial or erythropoietic system. For hemoglobin synthesis, plasma iron is delivered to the normoblast, where it remains up to 4 months, functioning in the hemoglobin molecules of a mature red cell. Senescent red cells then deteriorate. The iron is released from the hemoglobin by the reticuloendothelial system to reenter the transport pool for recycling. The normal iron distribution in a 70-kg adult (male) totals approximately 3.7 g, more than 65% in circulating hemoglobin. Another 27% is found in the storage pool as hemosiderin or ferritin. The body normally conserves iron so well that loss, usually only through the feces, is normally limited to about 1 mg/day. This amount is easily provided by a dietary intake of only 10 mg/day. Iron deficiency may follow extended intervals of inadequate iron intake (especially in women), in pregnancy when higher levels of iron are needed, or with excessive blood loss. Iron overload sometimes occurs in disorders in which normal regulation of iron absorption is impaired, called hemochromatosis. Hemochromatosis may be inherited or acquired in long-term transfusion therapy for chronic anemias. See also anemia, hemochromatosis, iron-deficiency anemia, iron transport.
