Study of Eryttirocyte Aggregation by Blood Viscometry at Low Shear Rates Using a Balance Method

A balance method was used to investigate the viscometric behavior and aggregation properties of blood at very low shear rates. The system was essentially a parallel-plate viscometer, consisting of a thin brass plate sus-pended by a fine wire from one end of an analytical beam balance. The yield shear stress of a sample was obtained directly by recording stress relaxation curves. Results were obtained for homogeneous Newtonian and non-Newtonian fluids and for blood and various red cell suspensions. It was found that human and canine blood behave as fluids exhibiting a yield stress. Suspensions of red cells in saline had much smaller values of yield stress, which is consistent with greatly diminished erythrocyte aggregation when fibrinogen is lacking. Meas-ured values of the yield stress were in the range of 0.001 to 0.1 dynes/cm2. ADDITIONAL KEY WORDS blood rheology parallel plate viscometer blood flow distribution microcirculation non-Newtonian flow yield shear stress hematocrit pulmonary circulation • It has been recognized since the time of PoiseuUle that blood is a complex fluid rheo-logically. Studies (1-4) have since shown that blood behaves in vitro as a non-New-tonian fluid whose viscosity may vary with the flow geometry and flow conditions. Sim-ilarly, the tendency of erythrocytes to aggre-gate in normal human blood has been well known since the work of Fahraeus (5) and is the basis of the test of sedimentation rate. However, only recently has work been under-taken to obtain quantitative rheological meas-urements in the low-shear region, where red cell aggregation is important (6-11). In par-ticular, experiments with Couette and capil-lary viscometers by Merrill and Cokelet and their co-workers (10-13) have indicated that the plasma protein fibrinogen causes normal human blood to exhibit a measurable yield shear stress under no-flow conditions. Although intercellular forces may dramat-ically affect the rheological behavior of From the Faculte de Medecine de Nancy, Physio