The rheology of human blood: A structured fluid approach based on rouleau behavior

The non-Newtonian, viscoelastic behavior of human blood was analyzed by studying the dynamic response of red-blood-cell aggregates (rouleaux) to mechanical stress. A new microcapillary device directly recorded the dynamic response of rouleaux to applied stresses. A rouleau was aspirated and pinioned by a 2 micron ID capillary. The pinioned rouleau was put under tension by hydrodynamic drag from an external flow field and its dynamic response was visualized through the optical microscope. Drag force calibration experiments gave a relationship between the applied drag force and other system variables. Rouleaux behaved as elastic springs when tension was applied but ruptured when subjected to excessive tension. The measured linear spring-constant for a rouleau from normal blood was approximately 5 ;The model's predicted semi-logarithmic dependence of stress versus time was followed very closely in low-shear-rate stress growth experiments. This technique appears to be superior to the semi-quantitative erythrocyte sedimentation rate test for assessing aggregation tendencies between red cells. Experiments which enhanced the rouleau adhesive strength (addition of 250,000 molecular weight Dextran to the plasma) suggest that the new microcapillary technique shows promise as a diagnostic test for pathologies exhibiting abnormal rouleau adhesive strengths such as diabetes mellitus, multiple myeloma, and Waldestrom's Macroglobulinemia.;A linear model attributed the viscoelastic behavior of blood to the linear-spring response of rouleaux and attributed the non-Newtonian behavior of blood to the changing size distribution of the rouleaux as they ruptured or aggregated in response to prevailing flow conditions. Independent measurements of aggregation kinetics (from light scattering) and of relaxation times (from rheometry) confirmed the predicted reciprocal relationship between the aggregation rate constant and zero-shear-rate limiting relaxation time. Measured spring constants (using the microcapillary technique) and spring constants computed from the predicted dependence of the zero-shear-rate limiting viscosity on measurable rouleau physical properties agreed within the limits of accuracy of the measurement of these properties.