Iron in the blood

The total iron content in the human body is approximately 4.2 g. About 75-80% of the total amount of iron is a part of hemoglobin, 20-25% of iron is reserve, 5-10% are part of myoglobin, 1% is contained in respiratory enzymes, catalyzing respiration processes in cells and tissues. Iron performs its biological function, mainly in the composition of other biologically active compounds, mainly enzymes. Iron enzymes perform four main functions:

• transport of electrons (cytochromes, iron-speroproteins);
• transport and deposition 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).

The homeostasis of iron in the body is ensured, first of all, by regulation of its absorption in connection with the limited ability of the organism to isolate this element.

There is a pronounced inverse relationship between the provision of the human body with iron and its absorption in the digestive tract. The absorption of iron depends on:

• age, iron availability of the organism;
• condition of the digestive tract;
• quantity and chemical forms of incoming iron;
• quantity and forms of other food components.

Reference values of iron concentration in blood serum

Age	Concentration of iron in serum
	μg / dL	μmol / l
Newborns	100-250	17.90-44.75
Children under 2 years	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

For optimal absorption of iron, normal secretion of gastric juice is necessary. The intake of hydrochloric acid facilitates the assimilation of iron in the case of achlorhydria. Ascorbic acid, reducing iron and forming with it chelate complexes, increases the availability of this element, as well as other organic acids. Another component of food that improves iron absorption is the "animal protein factor". Improving the absorption of iron simple carbohydrates: lactose, fructose, sorbitol, as well as amino acids such as histidine, lysine, cysteine, which form easily absorbed chelates with iron. Absorption of iron reduces such beverages as coffee and tea, the polyphenolic compounds of which firmly bind this element. Therefore, tea is used to prevent increased iron absorption in patients with thalassemia. A great influence on the absorption of iron has various diseases. It increases with iron deficiency, anemia (hemolytic, aplastic, pernicious), hypovitaminosis В ₆ and hemochromatosis, which is explained by increased erythropoiesis, depletion of iron stores and hypoxia.

Modern ideas of iron absorption in the intestine allot to a central role for two kinds of transferrin - mucous and plasma. Mucosal apotransferrin is secreted by enterocytes into the lumen of the intestine, where it combines with iron, and then enters the enterocyte. In the latter, he is freed from iron, and then 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 iron to its plasma analogue. In the cytosol of the enterocyte, some iron is included in ferritin, most of it is lost when the mucous membrane cells are clotted every 3-4 days, and only a small part passes into the blood plasma. Before being included in ferritin or transferrin, ferrous iron is converted into a trivalent iron. The most intense absorption of iron occurs in the proximal parts of the small intestine (in the duodenum and lean). Plasma transferrin delivers iron to tissues that have specific receptors. The inclusion of iron in the cell is preceded by the binding of transferrin by specific membrane receptors, in the loss of which, for example in mature erythrocytes, the cell loses its ability to absorb this element. The amount of iron entering the cell is directly proportional to the number of membrane receptors. The cell releases iron from transferrin. Then the plasma apotransferrin returns to circulation. Increasing the need for cells in the gland with their rapid growth or the synthesis of hemoglobin leads to the induction of the biosynthesis of transferrin receptors, and on the contrary, with an increase in iron stores 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 incorporated into the heme and other compounds.

In the human body there is a constant redistribution of iron. Quantitatively, the metabolic cycle is of the greatest importance: plasma → red bone marrow → erythrocytes → plasma. In addition, there are cycles: plasma → ferritin, hemosiderin → plasma and plasma → myoglobin, iron-containing enzymes → plasma. All these three cycles are interconnected through the iron of plasma (transferrin), which regulates the distribution of this element in the body. Usually 70% of the 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 higher than the intake of iron from the digestive tract (1-2 mg / day). More than 95% of the iron enters the plasma from the system of mononuclear phagocytes, which absorb by phagocytosis more than 10 ¹¹ old erythrocytes per day. Iron, which enters the cells of mononuclear phagocytes, either quickly returns to circulation in the form of ferritin, or is stored in reserve. Intermediate iron exchange is primarily associated with the processes of synthesis and decomposition of Hb, in which the system of mononuclear phagocytes plays a central role. In an adult human in the bone marrow, iron transferrin with specific receptors is included in normal cells and reticulocytes, which use it for hemoglobin synthesis. Hemoglobin, entering the blood plasma during the decay of erythrocytes, specifically binds to haptoglobin, which prevents its filtration through the kidneys. The iron released after the decomposition of hemoglobin in the mononuclear phagocyte system is again associated with transferrin and enters a new cycle of hemoglobin synthesis. In other tissues, transferrin delivers 4 times less iron than red bone marrow. The total iron content in the hemoglobin composition is 3000 mg, myoglobin contains 125 mg of iron, in the liver - 700 mg (mainly in the form of ferritin).

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

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

[1], [2], [3], [4], [5]