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Abstract

In the past, mammalian blood was characterized by flow curves to calculate the dynamic shear viscosity of blood. It was found out that whole blood viscosity (WBV) is very divers among the animal kingdom, predominantly at low shear stresses that allow red blood cell (RBC) aggregation. RBC aggregation might be the most significant factor for blood elasticity as well. To verify this hypothesis, we tested whole blood from species with high (horse), medium (man) and low (sheep) RBC aggregability by small amplitude oscillation in CSS-mode. Blood samples were hematocrit (HCT) adjusted (40%, 50%, 60%) and tested at 7°C, 22°C and 37°C. Storage modulus (G´) increased with RBC aggregability and HCT, but decreased with temperature, as expected. Interestingly, the gradient of the G´-increase with HCT was species-specific. The lower dependency of G´ on the equine HCT value could be a benefit during physical performance when high numbers of RBCs are released from the spleen. In sheep, a HCT-threshold had to be overcome before elasticity of the blood sample could be measured, suggesting that the cohesive forces between RBCs, and between RBCs and plasma molecules must be very low. The frequencies for tests under quasi-static condition were in a narrow range around the physiologic heart rate of the species. In horse, time-dependent influences concurred at frequencies lower than 3 rad.s-1 probably due to sedimentation of RBC aggregates. In conclusion, elasticity of blood depends not only on the amount of blood cells, but also on their mechanical and functional properties.

Keywords: Hemorheology; Blood; Species differences; Sheep; Horse; Small amplitude oscillation; Elasticity; Yield point; Storage modulus