книги студ / Color Atlas of Pathophysiology (S Silbernagl et al, Thieme 2000)

Plate 3.2 Erythrocytes; Erythropoiesis, Anemia

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abnormal erythropoiesis in its various steps (→ A), a shortened life-span, or chronic blood loss, can be differentiated by means of a number of diagnostic parameters:

Stem cells obtained by bone marrow puncture can be stimulated to proliferate and differentiate by erythropoietin in a cell culture. Colonies of more or less differentiated, hemoglo- bin-containing cells (E) are formed in this way (burst-forming units [BFU-E] or colony-forming units [CFU-E]). Their number is decreased if the anemia is caused by abnormal cell formation; it is increased if the cells are lost in a late stage of differentiation (erythroblast, erythrocyte) (→ A1).

Erythroblasts can be morphologically identified and quantified in a stained bone marrow sample. They decrease in number in aplasia and in defects of stem cell differentiation; they increase if erythropoiesis is stimulated, for example, by increased hemolysis (→ A2).

The efficiency of the entire erythropoiesis can be measured by determining the number of re-

32 ticulocytes (→ p. 30). If the number of reticulocytes is reduced, one must assume an abnor-

mality of cell formation (→ A3) because the second, theoretically possible cause, a prolongation of RBC life-span, does not occur. On the other hand, a longer lasting increase in reticulocyte numbers (reticulocytosis) is evidence for a chronically shortened life-span in the circulation on the part of the RBCs (chronic bleeding or hemolysis). Transitory reticulocytosis is a sign of stimulated erythropoiesis, for example, after acute blood loss, after acute hemolysis, or after correction of abnormal cell formation (with a high level of erythropoietin;

→ B2,3).

When erythrocytes are broken down in macrophages (→ p. 30), bilirubin, formed from liberated heme, is excreted in the bile after conjugation in the liver. The concentration of unconjugated (“indirect”) bilirubin in serum is increased in hemolysis (→ A4 and p.164ff.), but in some circumstances also if hemoglobin turnover is increased as a result of ineffective erythropoiesis.

The life-span of RBCs (shortened in hemolytic anemia; → A5) as well as their total volume can be measured by marking the erythrocytes in vitro with radioactive 51Cr (binding Cr to the Hb-β chain) and then re-infusing them. As 51Cr is released in hemolysis and then excreted by the kidneys, the erythrocyte life-span can be calculated from the loss of radioactivity measured daily. Total erythrocyte volume can be determined from the amount of 51Cr injected and the initial 51Cr concentration

in blood, using the principle of indicator dilution.

Measuring erythropoietin (→ A6). Lowered concentration of plasma erythropoietin suggests the anemia is caused nephrogenically (→ B4). However, most anemias are associated with a (compensatory) increase in erythropoietin concentration (→ B2,3).

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Erythrocytes (RBCs) serve to transport O2 and CO2 and also as a buffer. Hemoglobin (Hb) is essential for all three functions. It is composed of

concentration of intermediary products. While the rate of heme production is thus increased, these metabolites cause other disorders, namely porphyrias (→ p. 254).

Disorders of globin synthesis. Normally Hb is made up of 2 α chains of 141 amino acids each and 2 β chains of 146 amino acids (HbA1 = HbAα2β2). Only 2– 3% of Hb contains so-called δ-chains (HbA2 = Hbα2δ2) instead of the β-chains. Before birth a form of Hb is formed that has a higher O2 affinity (adaptation to a lower Po2 in the placenta). This fetal Hb (HbF) contains so-called γ-chains (Hbα2γ2) instead of the β-chains.

The properties of Hb (solubility, O2 affinity, oxidizability, etc.) are dependent upon the particular amino acid sequence. However, most of the over 300 genetically-determined Hb variants which have been indentified so far do not signficantly impair function. On the other hand, even a single “false” amino acid (valine instead of glutamate in position 6 in the β- chain = HbS; → A2) can lead to extensive functional disorders, as seen in sickle cell anemia,

36 which is caused by a homozygous gene defect. In the deoxygenated form, HbS aggregates in a

way that results in sickle-shaped erythrocytes (→ A). These sickle cells cannot be further deformed and get stuck inside the capillaries, causing occlusion of smaller blood vessels. Aggregation of HbS takes a few minutes so that it is especially those capillaries through which the blood flows slowly which are affected (spleen; vasa recta of the renal medulla; → p.106). If blood flow is slowed in general (shock) or if hypoxia occurs (at high altitude, during a flight, anesthesia), the abnormalities can spread to other organs (e.g., to the heart). Occlusion of the blood vessels further slows down blood supply in the affected regions and the Po2 is further reduced, so that a vicious circle results (crisis). Sickle cell anemia occurs nearly exclusively in blacks who themselves, or whose forbears, come from regions of Central Africa with a high prevalence of malaria. “Survival” of the defective gene in 40% of the population in Central Africa, despite the fact that until recently the disease was fatal in homozygous children, can be explained by the fact that heterozygous gene carriers are protected against the dangerous forms of malaria (selective advantage).

In β-thalassemia the production of β-chains is restricted, thus leading to a deficiency of HbA. It can be only partly compensated by an increased production of HbA2 and HbF. The incorporation of Fe2+ is diminished so that it remains in the erythrocytes (sideroachresia) and may accumulate excessively in the body (secondary hemochromatosis; → p. 252). Although the RBCs’ osmotic resistance (→ p. 40) is actually increased, their mechanical vulnerability is increased (rapid breakdown in the spleen, early hemolysis). While the heterozygous form (T. minor) causes few symptoms, the homozygous form (T. major) may be fatal even before puberty. The rare α-thalassemia usually causes death of the fetus, because without α- chains no HbF can be formed either. Hbγ4, produced in the fetus, and Hbβ4, occurring postnatally, are apparently inadequate substitutes for the normal Hb forms.

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Iron Deficiency Anemias

Of the iron (Fe) content in the body (2 g in females, 5 g in males) ca. 2⁄3 is bound to hemoglobin (Hb), 1⁄4 is stored iron (ferritin, hemosiderin), the rest is iron with diverse functions (myoglobin, Fe-containing enzymes). Loss of iron is ca. 1 mg/d in males and up to 2 mg/d in females (menstruation, pregnancy, birth). Of Fe taken up in food, 3– 15% is absorbed in the duodenum (→ A); in cases of Fe deficiency it can be up to 25% (see below). Iron intake with food should therefore be at least 10– 20 mg/d (women > children > men).

Iron absorption (→ A1). Fe can be absorbed relatively efficiently as heme-Fe2+ (found in meat and fish). The Fe (split off from heme) gets into the blood or remains in the mucosa as ferritin-Fe3+ and returns to the lumen on mucosal cell disintegration. Non-heme Fe can be absorbed only in the form of Fe2+, which is absorbed by a Fe2+-H+-symport carrier (DCT1) (in competition with Mn2+, Co2+, Cd2+, etc.). A low pH of the chyme is essential for absorption, because it will 1) increase the H+ gradient that drives Fe2+ into the cell via DCT1, and 2) release Fe from compounds in food. Non-heme Fe3+ in food must be reduced by ferrireductase (+ascorbate) to Fe2+ on the surface of the luminal mucosa (→ A1, FR). Fe uptake by blood is regulated by the intestinal mucosa: in Fe deficiency mucosal ferritin translation is inhibited by binding the Fe-regulating protein IRP1 to ferritin-mRNA, so that more of the absorbed Fe2+ can reach the blood. There it is oxidized by ceruloplasmin (+copper) to Fe3+ and bound to apotransferrin, which transports Fe in plasma (→ A). Transferrin (= apotransferrin with 2 Fe3+) is taken up, via transferrin receptors, endocytotically in erythroblasts and in hepatic, placental, and other cells. After Fe has transferred to the target cells, apotransferrin again becomes available for Fe absorption from the intestine and macrophages (see below).

Iron storage (→ A2). Ferritin (in the intestinal mucosa, liver, bone marrow, erythrocytes,

and plasma), which has a “pocket” for 4500 Fe3+ ions, is a rapidly available iron reserve (ca. 600 mg), while Fe from hemosiderin is more difficult to mobilize (250 mg Fe in macrophages from liver and bone marrow). Hb-Fe and heme-Fe, released from malformed erythroblasts (so-called inefficient erythropoiesis) and hemolyzed erythroblasts, is bound to haptoglobin and hemopexin respectively, and taken up by the macrophages in bone marrow or by liver and spleen by endocytosis, 97% being reused.

Iron deficiency (serum Fe < 0.4 mg/L; serum ferritin ↓) inhibits Hb synthesis (→ p. 36) so that hypochromic microcytic anemia develops: MCH < 26 pg, MCV < 70 fL, Hb < 110 g/L. Its causes are (→ A and Table):

Blood loss (gastrointestinal tract, increased menstrual bleeding) in adults is the most common cause of iron deficiency (0.5 mg Fe lost with each mL of blood).

Fe recycling is decreased; this form of anemia (the second most common worldwide) occurs with chronic infections. In this situation the Fe regained by the macrophages is no longer adequately released and thus cannot be reused.

Fe uptake is too low (malnutrition, especially in the developing countries).

Fe absorption is reduced due to: 1) achlorhydria (atrophic gastritis, after gastrectomy; → p.142, 148); and 2) malabsorption in diseases of the upper small intestine or in the presence of Fe-binding food components (phytate in cereals and vegetables; tannic acid in tea, oxalates, etc.).

There is increased Fe requirement (growth, pregnancy, breast-feeding).

An apotransferrin defect (rare).

If Fe overloading occurs in the body, damage

is caused mainly to the liver, pancreas and myocardium (hemochromatosis) (→ p. 252).

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