Iron forms the nucleus of the iron-porphyrin heme ring, which together with globin chains forms hemoglobin. Hemoglobin reversibly binds oxygen and provides the critical mechanism for oxygen delivery from the lungs to other tissues. In the absence of adequate iron, small erythrocytes with insufficient hemoglobin are formed, giving rise to microcytic hypochromic anemia. Iron-containing heme is also an essential component of myoglobin, cytochromes, and other proteins with diverse biologic functions.
Free inorganic iron is extremely toxic, but iron is required for essential proteins such as hemoglobin; therefore, evolution has provided an elaborate system for regulating iron absorption, transport, and storage (Figure 33–1). The system uses specialized transport, storage, ferrireductase, and ferroxidase proteins whose concentrations are controlled by the body’s demand for hemoglobin synthesis and adequate iron stores (Table 33–1). A peptide called hepcidin, produced primarily by liver cells, serves as a key central regulator of the system. Nearly all of the iron used to support hematopoiesis is reclaimed from catalysis of the hemoglobin in senescent or damaged erythrocytes. Normally, only a small amount of iron is lost from the body each day, so dietary requirements are small and easily fulfilled by the iron available in a wide variety of foods. However, in special populations with