The human microcirculation is taken to include all vessels with internal diameter less than 500 μm. In these small vessels the heterogeneous nature of the blood suspension becomes apparent in the external characteristics of the flow, which signifies that a continuum model of blood flow in this regime is inadequate. The motion and deformation of red cells in the capillaries is discussed in detail with emphasis on the relationship between red cell dynamics and apparent viscosity. Large scale hydraulic models of red cell motion in capillaries have yielded dimensionless correlations applicable to the microscopic prototypes. In the larger vessels the distribution of red cells across the vessel lumen is generally nonuniform and axial velocity profiles reveal the occurrence of partial plug flow. Red cells traveling near the vessel wall where the shear rate is highest may exhibit a transition to liquid droplike behavior.

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