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Pathology > Basic Hematology > Normal Hematopoiesis > Erythrocyte Review > Iron Metabolism

Iron Metabolism

Iron is critical to a number of synthetic and enzymatic processes. Most of the body iron is part of the hemoglobin molecule where iron serves a key role in oxygen transport. Iron is recycled and thus conserved by the body. Daily intake ( 1 mg ) s balanced against small daily losses (1 mg ). The Fe button below will take you to the iron cycle, allowing you to see the entire cycle as a whole.

 

The amounts shown in the Fe Cycle Card are in mg of iron lost or gained per day. They were derived in the following manner.

The average blood volume in a 70 kg man is 5,000 ml.

There are 150 grams of hemoglobin in each liter of blood, therefore there are 750 g of hemoglobin in the body.

Each gram of hemoglobin contains approximately 3.3 mg of iron or 2475 mg of iron in the body.

Dividing the 2475 mg total by the 120 day average RBC lifespan results in the iron needed per day or 20.6 mg iron/day.

Hemoglobin Fe

2200 mg

Ferritin & Hemosiderin

1000 mg

Myoglobin Fe

300 mg

Other Fe (cytochromes; enzymes)

100 mg

Total Body Iron

3600 mg

An average adult in the U.S. on a 2,500 calorie diet ( 6 mg of iron/1,000 kcal) ingests 15 mg of iron daily. Only 5-10 % or about 1.0 mg of dietary iron is absorbed as ferrous iron (Fe++), mainly in the duodenum and upper jejunum where the pH is low. The mucosal cells oxidize the ferrous iron to ferric iron , which is then complexed with apoferritin to form ferritin. Some of the ferritin is transported out of the mucosal cell into the plasma bound to transferrin. Thus bound, iron can be transported to the bone marrow or iron storage sites where it is stored as either ferritin or hemosiderin.

 

 

 

 

Most cells have transferrin receptors (CD 71) to which iron ladden transferrin binds. The receptor-transferrin-iron complex is then incorporated into the cytosol by endocytosis. In red cells the endocytotic vacuole fuses with a lysozyme, where at an acid pH the iron (Fe++) is released from transferrin and transported to mitochondria where it is incorporated into heme, the ferrous iron complex of protoporphyrin IX.

Although iron is utilized in virtually all cells, the bulk of body iron is found in erythrocytes with lesser amounts in myoglobin. Large amounts of iron are required during growth periods in infant, childhood and teenage years.

Transferrin carries iron to the bone marrow where it is accepted into RBCs via a transferrin receptor (CD71) and incorporated into heme for use in hemoglobin.

Not all erythrocytes develop and mature successfully. Some die in the marrow and their iron is salvaged by macrophages. This failure to mature resulting in death in the marrow is known as ineffective erythropoiesis.

Normally only small amounts of iron are lost daily as hair, skin, urinary bladder,and gastrointestinal cells are shed. This amount can easily be replaced by dietary intake.

With bleeding, larger amounts of iron can be lost. The most common normal blood losses are due to menstruation and pregnancy.

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