[Microrheology of blood in capillaries (author's transl)]

Even in the presence of red blood cells the viscous resistance of blood flow through narrow capillaries is only some 15-20% higher than that of pure plasma flow. The uniquely low "hydrodynamic disturbance effect" of the erythrocytes results from their similarly unique flow properties which allow them to adapt to capillary flow conditions by viscous deformation, orientation stability, and membrane rotation. In addition, the volume concentration of red cells decreases with decreasing vessel diameter, with the result that the majority of capillary vessels contains blood of an hematocrit which is less than 50% of the systemic value.