Rheogoniometric studies of whole human blood at shear rates down to 0.0009 sec −1

The rheological properties of whole blood from healthy human subjects were measured over a wide range of shear rates from 1000 down to 0.0009 sec⁻¹. A rheological equation of state is proposed to characterize the flow properties of blood. The flow curves were divided into three regions of shear rates. In region I (between 50 and 1000 sec⁻¹) the blood exhibits nearly Newtonian behavior and the viscosity is not dependent on time of shearing. In region II (between 10⁻² and 50 sec⁻¹) the blood shows thixotropic behavior as predicted by the rheological equation. It is contended that the time-dependent behavior is caused by progressive desaggregation of aggregated erythrocytes, for which experimental evidence exists. In region III (between 10⁻³ and 10⁻² sec⁻¹) the associated shear stresses are considered to be below the yield stress of the blood. At these minimal shear rates the blood behaves as a solid and, therefore, the rheological equation of state for a fluid is not applicable. A tentative explanation is given for rheological behavior of blood in this region by using the basic principle of electrostatic stability between erythrocytes and by the possible occurrence of linkages at points along the chains of the rouleaux of erythrocytes, forming a three-dimensional structure.