Rheological characteristics of horse blood: significance during exercise.

When horses maximally exercise, splenic contraction and fluid movement out of the vascular compartment greatly increase the hematocrit (up to 0.70). We studied the in vitro rheological characteristics of blood from Thoroughbreds and Quarter Horses to determine the interaction of hematocrit and shear rate on apparent viscosity. We also compared the rheological characteristics of the blood before and after horses received furosemide, a drug commonly used to prevent exercise-induced pulmonary hemorrhage. Although the apparent viscosity of blood with a high hematocrit was high at low shear rates, it rapidly decreased as the shear rate increased and appeared to continue to decrease at shear rates above 450 sec-1, which was the limit of our measurement capability. Furosemide had no detectable influence on the measured in vitro rheological characteristics of the blood at any hematocrit or shear rate studied. We postulate that during exercise, when shear rates in the circulation are high, apparent viscosity at high hematocrit may approach values similar to those that occur during rest when both hematocrit and shear rates are lower. Consequently, the shear-dependent properties of blood may create a homeostasis of viscosity in vivo during exercise so that high viscosity is not a major factor contributing to vascular resistance.