<P> Macrophages of the reticuloendothelial system store iron as part of the process of breaking down and processing hemoglobin from engulfed red blood cells . Iron is also stored as a pigment called hemosiderin which is an ill - defined deposit of protein and iron, created by macrophages where excess iron is present, either locally or systemically for example among people with iron overload due to frequent blood cell destruction and transfusions . If the systemic iron overload is corrected, over time the hemosiderin is slowly resorbed by macrophages . </P> <P> Human iron homeostasis is regulated at two different levels . Systemic iron levels are balanced by the controlled absorption of dietary iron by enterocytes, the cells that line the interior of the intestines, and the uncontrolled loss of iron from epithelial sloughing, sweat, injuries and blood loss . In addition, systemic iron is continuously recycled . Cellular iron levels are controlled differently by different cell types due to the expression of particular iron regulatory and transport proteins . </P> <P> The absorption of dietary iron is a variable and dynamic process . The amount of iron absorbed compared to the amount ingested is typically low, but may range from 5% to as much as 35% depending on circumstances and type of iron . The efficiency with which iron is absorbed varies depending on the source . Generally the best - absorbed forms of iron come from animal products . Absorption of dietary iron in iron salt form (as in most supplements) varies somewhat according to the body's need for iron, and is usually between 10% and 20% of iron intake . Absorption of iron from animal products, and some plant products, is in the form of heme iron, and is more efficient, allowing absorption of from 15% to 35% of intake . Heme iron in animals is from blood and heme - containing proteins in meat and mitochondria, whereas in plants, heme iron is present in mitochondria in all cells that use oxygen for respiration . </P> <P> Like most mineral nutrients, the majority of the iron absorbed from digested food or supplements is absorbed in the duodenum by enterocytes of the duodenal lining . These cells have special molecules that allow them to move iron into the body . To be absorbed, dietary iron can be absorbed as part of a protein such as heme protein or iron must be in its ferrous Fe form . A ferric reductase enzyme on the enterocytes' brush border, duodenal cytochrome B (Dcytb), reduces ferric Fe to Fe . A protein called divalent metal transporter 1 (DMT1), which can transport several divalent metals across the plasma membrane, then transports iron across the enterocyte's cell membrane into the cell . </P>

Where does iron in your blood come from