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

The high cardiac output, arterial blood pressure and cardiac energy de mand of tuna make it likely that blood viscosity has an important infl uence on cardiovascular function. Furthermore, tuna regularly subject themselves to ambient temperature changes of 10 degrees C or more duri ng their daily vertical migrations. They can also maintain muscle temp eratures several degrees Celsius above ambient and reach maximum muscl e temperatures of approximately 15 degrees C above ambient. The blood of tuna is, therefore, subjected to more frequent and rapid temperatur e changes than those that occur in other teleosts. Nothing is known, h owever, about the effects of temperature, shear rate or hematocrit on the viscosity of tuna blood. Viscosity of yellowfin tuna blood (hemato crits of 0-55%) was measured at 15, 25 and 35 degrees C and at shear r ates of 45, 90, 225 and 450 s(-1) using a cone-plate viscometer. As fo und for the blood of other vertebrates, viscosity increased with incre asing hematocrit. Viscosity also increased nonlinearly with decreasing shear rate, until a shear rate of 90s(-1) was reached. There was no s ignificant increase in viscosity when shear rate was decreased further , to 45 s(-1). Because of the relatively flat hematocrit-viscosity cur ves, predicted optimal hematocrit curves were nearly flat above a hema tocrit of approximately 30%.