Optimal hematocrit theory during activity in the bullfrog (Rana catesbeiana)

Area of Science:

• Physiology
• Biophysics
• Herpetology

Background:

• Amphibian blood exhibits non-Newtonian fluid properties, with viscosity dependent on shear rate and a finite yield shear stress.
• Understanding blood viscosity and hematocrit is crucial for physiological studies, particularly in ectothermic vertebrates like bullfrogs.

Purpose of the Study:

• To investigate the relationship between blood viscosity and hematocrit in bullfrogs (Rana catesbeiana).
• To determine the in vitro and in vivo optimal hematocrit for bullfrog blood flow.
• To assess the impact of elevated hematocrit on blood pressure and systemic blood flow.

Main Methods:

• An exponential relationship (eta = 1.81 e0.033Hct) was established between blood viscosity (eta) and hematocrit (Hct) in bullfrog blood.
• Isovolumic erythrocythemia was used to increase bullfrog hematocrit from 27% to 57%.
• Systemic arch blood flow and blood pressure were measured in bullfrogs with experimentally elevated hematocrit.

Main Results:

• An in vitro optimal hematocrit of 30% was calculated based on the viscosity-hematocrit relationship.
• Elevated hematocrit led to a slight increase in systolic, diastolic, and venous blood pressure.
• Systemic arch blood flow was inversely related to blood viscosity, with an in vivo optimal hematocrit of 41% determined.

Conclusions:

• Optimal hematocrit theory is applicable to bullfrogs (Rana catesbeiana) both in vitro and in vivo.
• The decrease in blood flow at high hematocrits was primarily due to reduced pulse volume.
• Variability in optimal hematocrit among individuals may reflect methodological factors or physiological compensation mechanisms.