Blood advancing in living capillaries (5 to 10 m in diameter) and in the adjoining arterioles and venules (10 to 25 m in diameter) itself represents a specific substance hardly comparable with fluids in a usual understanding of this term. Its greatest part comprises the deformed red blood cells (RBCs) whose size is similar to luminal diameters of the microvessels. The RBCs (comprising great majority of the forming elements in microvessels) are disposed in the normal flow not chaotically, but in

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... a certain order that was identified therefore as "blood flow structure (or struc-turing) in microvessels" [1] [2] [3] . The specific regime of the RBCs flow in blood vessels changes from larger to smaller luminal diameters. Thus in vessels larger than 0.2 mm, the blood flow can be treated as a homogenous suspension with little error. By contrast, in arterioles or venules smaller than 25 m, and especially in capillaries (whose diameters are smaller than 8 m in humans), the RBCs are not uniformly dispersed, especially because of the presence of a parietal plasma layer containing no cells; thus the blood flow is specifically structured (Fig. 1) . Disorders of this kind inevitably lead to the disturbance of normal blood rheological properties in the microvessels. Fig. 1. Blood flow in microvascular region. A marginal cell-free (plasma) layer is formed by the axial migration of RBCs in microvessels with multifile flow and even by the deformation of RBCs in capillaries with single-file flow. A flow condition at a branching point provides plasma skimming (plasma flow without RBCs). Blood Flow Structuring in Microvessels Fig. 2. Principal features of blood flow structure in narrow microvessels (capillaries and the adjacent arterioles and venules). Fig. 5. Schematic demonstration of the effect of RBC aggregation on blood rheological properties and blood flow in microvessels.