Exchange of iron is normal

Normally a person's daily diet contains about 10-20 mg of iron (90% in the free state, 10% in conjunction with the heme), of which 1-1.5 mg is absorbed. The amount of absorbed iron depends on its stores in the body: the higher the demand, the more iron is absorbed. Absorption occurs in the upper parts of the small intestine and is an active process in which iron can be transferred even against the concentration gradient. However, the transport mechanisms are unknown. Proteins that can be carriers of iron have been identified, but their exact role has not yet been established.

In the cells of the mucosa, iron is in the cytosol. Some of it is bound and stored in the form of ferritin, which is subsequently either used or lost as a result of cell sloughing. Part of the iron, intended for metabolism in other tissues, is transported through the basolateral membrane of the cell (the mechanism is not studied) and binds to transferrin, the main transport protein of iron in the blood.

Transferrin (molecular weight 77 000 Da) is a glycoprotein, synthesized mainly in the liver. It can bind two iron molecules. The total iron-binding capacity of serum due to transferrin is 250-370 μg%. Normally, transferrin is saturated with iron by about a third. The physiological absorption of iron by reticulocytes and hepatocytes depends on transferrin receptors on the cell surface, which have an affinity mainly to transferrin associated with iron. The iron complex with the receptor enters the cell where iron is released. When the cell is saturated with iron, the cellular transferrin receptors are inhibited. When complete transferrin saturation is observed, for example, in severe hemochromatosis, iron circulates in forms not associated with transferrin as compounds with low molecular weight chelators. In this form, iron easily enters the cells regardless of the degree of saturation with iron.

In cells, iron is deposited in the form of ferritin (a molecular weight of 480,000 Da), a complex of the apoferritin protein (subunits of H and L) with iron, which in electron microscopy has the form of particles with a diameter of 50 A, freely located in the cytoplasm. One ferritin molecule can contain up to 4,500 iron atoms. At high iron concentrations, the synthesis of apoferritin is enhanced.

Accumulations of decomposed ferritin molecules are hemosiderin, which is colored blue with ferrocyanide. About a third of the iron stores in the body are in the form of hemosiderin, the amount of which increases with diseases associated with excessive accumulation of iron.

Lipofuscin , or wear pigment, accumulates due to iron overload. It has a yellow-brown color and does not contain iron.

As the need for hemoglobin synthesis increases, iron deposited in the form of ferritin or hemosiderin is mobilized.

Normally, the body contains about 4 g of iron, of which 3 g in the hemoglobin, myoglobin, catalase and other respiratory pigments or enzymes. Stocks of iron are 0.5 grams, of which 0.3 g are in the liver, but with conventional histological examination with iron staining using conventional methods, they are not visible. The liver is the main storage site for iron absorbed in the intestine. At its maximum saturation, iron is deposited in other parenchymal organs, including acinar cells of the pancreas and cells of the anterior lobe of the pituitary gland. The reticuloendothelial system plays a limited role in the accumulation of iron and becomes the site of preferential deposition of iron only with its intravenous administration. Iron from the destroyed red blood cells accumulates in the spleen.

[1], [2], [3], [4], [5], [6], [7], [8], [9]