[Hemoglobin S as the cause of primary hyperviscosity of the blood]

The hemoglobinemia S is a genetic defect due to a replacement of a single amino acid in the beta-chain of the human hemoglobin, leading to the most characteristic case of primary blood hyperviscosity. The difference in beta-chain of normal HbA compared to that of HbS is represented by the replacement of glutamic acid, normally found in the 6th position, with valine. The sickling of HbS-containing erythrocytes is due to the polymerization of deoxygenated HbS tetramers with formation of linear structures and to the parallel distribution of these fibers in the red blood cell. The full deoxygenation of HbS blood induces the characteristic morphological changes of red blood cell and increases the blood viscosity from 40 to 120% if compared to the viscosity of the same oxygenated blood. Any change in the viscosity of normal blood following deoxygenation was observed. Almost all the clinical symptoms found in patients carrying HbS can be directly or indirectly correlated to the increased blood viscosity following deoxygenation.