Iron in the blood

The total iron content in the human body is approximately 4.2 g. Approximately 75-80% of the total iron is found in hemoglobin, 20-25% of iron is in reserve, 5-10% is found in myoglobin, 1% is contained in respiratory enzymes that catalyze the processes of respiration in cells and tissues. Iron performs its biological function mainly as part of other biologically active compounds, primarily enzymes. Iron-containing enzymes perform four main functions:

• electron transport (cytochromes, iron-sulfur proteins);
• transport and storage of oxygen (hemoglobin, myoglobin);
• participation in the formation of active centers of oxidation-reduction enzymes (oxidases, hydroxylases, SOD, etc.);
• transport and deposition of iron (transferrin, hemosiderin, ferritin).

Iron homeostasis in the body is ensured, first of all, by the regulation of its absorption due to the limited ability of the body to excrete this element.

There is a clear inverse relationship between the human body's iron status and its absorption in the digestive tract. Iron absorption depends on:

• age, iron status of the body;
• gastrointestinal tract conditions;
• the amount and chemical forms of incoming iron;
• quantities and forms of other food components.

Reference values for serum iron concentration

Age	Serum iron concentration
	Mcg/dl	µmol/l
Newborns	100-250	17.90-44.75
Children under 2 years old	40-100	7.16-17.90
Children	50-120	8.95-21.48
Adults:
Men	65-175	11.6-31.3
Women	50-170	9.0-30.4

Normal secretion of gastric juice is necessary for optimal absorption of iron. Taking hydrochloric acid promotes iron absorption in achlorhydria. Ascorbic acid, which reduces iron and forms chelate complexes with it, increases the availability of this element, as do other organic acids. Another food component that improves iron absorption is the "animal protein factor". Simple carbohydrates improve iron absorption: lactose, fructose, sorbitol, as well as amino acids such as histidine, lysine, cysteine, which form easily absorbed chelates with iron. Iron absorption is reduced by drinks such as coffee and tea, whose polyphenolic compounds firmly bind this element. Therefore, tea is used to prevent increased iron absorption in patients with thalassemia. Various diseases have a significant impact on iron absorption. It increases with iron deficiency, anemia (hemolytic, aplastic, pernicious), hypovitaminosis B6 _and hemochromatosis, which is explained by increased erythropoiesis, depletion of iron stores and hypoxia.

Modern concepts of iron absorption in the intestine assign a central role to two types of transferrin - mucosal and plasma. Mucosal apotransferrin is secreted by enterocytes into the intestinal lumen, where it combines with iron, after which it penetrates the enterocyte. In the latter, it is freed from iron, after which it enters a new cycle. Mucosal transferrin is formed not in enterocytes, but in the liver, from which this protein enters the intestine with bile. On the basal side of the enterocyte, mucosal transferrin gives up iron to its plasma analogue. In the cytosol of the enterocyte, some iron is included in ferritin, most of it is lost during the desquamation of mucosal cells, which occurs every 3-4 days, and only a small part passes into the blood plasma. Before being included in ferritin or transferrin, divalent iron is converted to trivalent. The most intensive absorption of iron occurs in the proximal parts of the small intestine (in the duodenum and jejunum). Plasma transferrin delivers iron to tissues that have specific receptors. The incorporation of iron into a cell is preceded by the binding of transferrin to specific membrane receptors, the loss of which, for example, in mature erythrocytes, the cell loses the ability to absorb this element. The amount of iron entering the cell is directly proportional to the number of membrane receptors. Iron is released from transferrin in the cell. Then plasma apotransferrin returns to circulation. An increase in the cells' need for iron during their rapid growth or hemoglobin synthesis leads to the induction of transferrin receptor biosynthesis, and vice versa, with an increase in iron reserves in the cell, the number of receptors on its surface decreases. Iron released from transferrin inside the cell binds to ferritin, which delivers iron to the mitochondria, where it is included in the composition of heme and other compounds.

In the human body, iron is constantly redistributed. In quantitative terms, the most important is the metabolic cycle: plasma → red bone marrow → erythrocytes → plasma. In addition, the following cycles function: plasma → ferritin, hemosiderin → plasma and plasma → myoglobin, iron-containing enzymes → plasma. All three of these cycles are interconnected through plasma iron (transferrin), which regulates the distribution of this element in the body. Typically, 70% of plasma iron enters the red bone marrow. Due to the breakdown of hemoglobin, approximately 21-24 mg of iron is released per day, which is many times greater than the intake of iron from the digestive tract (1-2 mg/day). More than 95% of iron enters the plasma from the mononuclear phagocyte system, which absorbs more than 10 ¹¹ old erythrocytes per day by phagocytosis. Iron that enters the cells of mononuclear phagocytes either quickly returns to circulation in the form of ferritin or is stored for future use. Intermediate iron metabolism is primarily associated with the processes of Hb synthesis and decay, in which the mononuclear phagocyte system plays a central role. In an adult, transferrin iron in the bone marrow is incorporated into normocytes and reticulocytes using specific receptors, which use it to synthesize hemoglobin. Hemoglobin entering the blood plasma during the decay of erythrocytes specifically binds to haptoglobin, which prevents its filtration through the kidneys. Iron released after the breakdown of hemoglobin in the mononuclear phagocyte system again binds to transferrin and enters a new cycle of hemoglobin synthesis. Transferrin delivers 4 times less iron to other tissues than to the red bone marrow. The total iron content in hemoglobin is 3000 mg, in myoglobin - 125 mg of iron, in the liver - 700 mg (mainly in the form of ferritin).

Iron is excreted from the body mainly by exfoliation of the intestinal mucosa and with bile. It is also lost with hair, nails, urine and sweat. The total amount of iron excreted in this way is 0.6-1 mg/day in a healthy man, and more than 1.5 mg in women of reproductive age. The same amount of iron is absorbed from food (5-10% of its total content in the diet). Iron from animal food is absorbed several times better than from plant food. Iron concentration has a daily rhythm, and in women there is a connection with the menstrual cycle. During pregnancy, the iron content in the body decreases, especially in the second half.

Thus, the concentration of iron in the serum depends on resorption in the gastrointestinal tract, accumulation in the intestine, spleen and red bone marrow, on the synthesis and breakdown of Hb and its loss by the body.

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