Once absorbed through the mucosal cells of the duodenum, iron is bound to a carrier plasma protein, transferrin (Tf), for movement to sites of utilization. Almost all iron in plasma is bound to Tf, and most Tf-bound iron is carried to the bone marrow to be incorporated into developing erythrocytes. Transferrin usually is about 20% to 40% saturated with iron.6
Transferrin releases iron to specific transferrin receptors (TfRs) for movement into cells. Transferrin receptors are found on all cells but are found in relatively high concentrations in erythroid precursors, hepatocytes (liver cells), and placental cells. When the capacity of plasma Tf to bind iron is exceeded, i.e., transferrin saturation (TS) is higher than normal, excess iron is taken up by hepatocytes and other cells.
A brief summary of this iron cycle is illustrated in the image to the right.