Rheology of the microcirculation.

The main function of the microvasculature is the controlled exchange of materials with surrounding tissues. This necessitates a large vessel surface established by a high number of vessels with small diameters and thus an inherently high individual resistance to flow. The hydrodynamic resistance of a microvascular network with given angioarchitecture depends on the apparent viscosity of blood flowing in the microvessels. Apparent viscosity declines with decreasing diameter (the Fahraeus-Lindqvist effect) and is minimal at diameters of about 5-7 micrometers due to the optimal alignment of red cells with the flow. In vivo, a number of additional phenomena influence blood rheology and network hemodynamics. The distribution of blood flow and red cell flux within networks is influenced by the mechanics of red cell motion at individual diverging bifurcations (phase-separation effect). Furthermore, recent studies have revealed the presence of a thick endothelial surface layer ( approximately 0.5 micrometers) on the luminal surface of microvessels which is attached to the endothelial glycocalyx. This layer modulates flow resistance and may be relevant for a number of other processes such as inflammatory responses and blood coagulation. Information on microvascular rheology can be used to develop mathematical models of network hemodynamics and vascular adaptation to the local environment (angioadaptation), to investigate the complex interrelated mechanisms which establish and maintain functionally adequate microvascular networks.