[Hemorheology, hemodynamics and microcirculation. 1].

The microcirculation constitutes an ubiquitous vascular network presenting a mesh pattern, and comprising different types of vessels, arterioles, small veins, capillaries, arteriovenous shunts or similar structures, and lymphatics. Many dimensions have to be recognized, or simply mentioned, if one is to understand the hemodynamic and hemorheological particulars of this territory, which differ, in many aspects, from those specific to the macrocirculation (number and length of the vessels, diameter and cross section, intercapillary distance, geometric characteristics, intravascular pressure, pressure gradient, pressure-volume relationship, flow rate, mean velocity of plasma and RBC, velocity profile, local hematocrit, in situ viscosity, kinematic viscosity, wall shearing conditions, local oxygen transport, aggregation and deformability of RBC, leukocyte properties, etc.). The flow rate in capillary tubes and capillary vessels of the living organism varies with many factors, such as proximal hemodynamics, hemorheological characteristics of blood (fibrinogen, macro- and micro-hematocrit), some known effects (Farheus, Farheus Lindqvist), local diameter, the plasma layer which plays the role of the limiting layer, the endothelial film, the wall effect, and so forth. Models of the circulation have been propounded, none of which takes into account the whole of these phenomena due to their great complexity. Hemodynamic and hemorheological interactions provide for a better understanding of certain concepts, such as vascular resistance, hindrance, capacitance, local flow rates, real capillary opening and closing, development of two-directional functional shunts, autoregulation, pressure-volume relationship, critical closing pressure, circulatory current slowing effect, sequelae of intravascular aggregation of formed blood elements.