THE INFLUENCE OF HEMATOCRIT, TEMPERATURE AND SHEAR RATE ON THE VISCOSITY OF BLOOD FROM A HIGH-ENERGY-DEMAND TELEOST, THE YELLOWFIN TUNA THUNNUS ALBACARES

The high cardiac output, arterial blood pressure and cardiac energy demand of tuna make it likely that blood viscosity has an important influence on cardiovascular function. Furthermore, tuna regularly subject themselves to ambient temperature changes of 10°C or more during their daily vertical migrations. They can also maintain muscle temperatures several degrees Celsius above ambient and reach maximum muscle temperatures of approximately 15°C above ambient. The blood of tuna is, therefore, subjected to more frequent and rapid temperature changes than those that occur in other teleosts. Nothing is known, however, about the effects of temperature, shear rate or hematocrit on the viscosity of tuna blood. Viscosity of yellowfin tuna blood (hematocrits of 0­55 %) was measured at 15, 25 and 35°C and at shear rates of 45, 90, 225 and 450 s-1 using a cone-plate viscometer. As found for the blood of other vertebrates, viscosity increased with increasing hematocrit. Viscosity also increased nonlinearly with decreasing shear rate, until a shear rate of 90 s-1 was reached. There was no significant increase in viscosity when shear rate was decreased further, to 45 s-1. Because of the relatively flat hematocrit­viscosity curves, predicted optimal hematocrit curves were nearly flat above a hematocrit of approximately 30 %.