The missing negative effect of red cell aggregation upon blood flow in small capillaries at low shear forces

Flow resistance of EDTA or defibrinized blood was measured in a vertical capillary viscometer. (Capillary I.D. = 0.3 mm, 1 = 475 mm, tau w = 0.27 to 0.036 Pa). With decreasing shear forces the apparent viscosity increased from 12 to 16 mPa X s, i.e. by about 30%. There was no significant difference in flow resistance between red cells suspended in plasma or serum. The apparent viscosity of the same blood sample increased from 14 to 45 mPa X s for RBC in plasma i.e. by about 320% if measured with a rotational viscometer. The attempt is made to explain this difference in flow resistance with the different effect of shear resistant RBC-aggregates upon a parabolic streaming profile in a capillary, or upon the quasi linear profile within the annular gap of a rotational viscometer.