The ferrous form of iron is more readily absorbed than ferric iron. Duodenal microvilli contain ferric reductase to promote absorbtion of ferrous iron. Only a small fraction of the body's iron is gained or lost each day. Most of the iron in the body is recycled when old red blood cells are taken out of circulation and destroyed, with their iron scavenged by macrophages in the mononuclear phagocyte system, and returned to the storage pool for re-use. Iron homeostasis is closely regulated via intestinal absorption.
Increased absorption is signalled by decreasing iron stores, hypoxia, and erythropoietic activity.
There are about 2 gm of iron in the adult female, and up to 6 gm iron in the adult male. About 1.5 to 2 gm of this total is found in red blood cells as heme in hemoglobin, and 0.5 to 1 gm occur as storage iron, with the remainder in myoglobin and in enzymes that require iron. The simplest tests that indirectly give an indication of iron stores are the serum iron and iron binding capacity, with calculation of the percent transferrin saturation. The serum ferritin correlates well with iron stores, but it can also be elevated with liver disease, inflammatory conditions, and malignant neoplasms. The amount of storage iron for erythropoiesis can be quantified by performing an iron stain on a bone marrow biopsy.