Separating the contributions of vascular anatomy and blood viscosity to peripheral resistance and the physiological implications of interspecific resistance variation in amphibians

Amphibian pulmonary and systemic vascular circuits are arranged in parallel, with potentially important consequences for resistance (R) to blood flow. The contribution of the parallel anatomic arrangement to total vascular R (R _T), independent of blood viscosity, is unknown. We measured pulmonary (R _P) and systemic (R _S) vascular R with an in situ Ringer’s solution perfusion technique using anesthetized anuran and urodele species to determine: (1) relative contributions of vascular anatomy and blood viscosity to R _T; (2) distensibility index (%Δ flow kPa^?1) of the pulmonary and systemic vascular circuits; and (3) interspecific correlates of variation in these parameters with red blood cell size, cardiac power output, and aerobic capacities. R _P was lower than R _S in anurans, while R _P of the urodeles was greater than R _S and significantly greater than anuran R _P. Anuran R _T was lowest and did not vary interspecifically, whereas urodele R _T was significantly greater than anuran, and varied interspecifically. Pulmonary and systemic circuit distensibility differences may explain cardiac shunt patterns in toads with changes in cardiac output from rest to activity. When blood viscosity was taken into account, vascular resistance accounted for about 25 % of R _T while blood viscosity accounted for the remaining 75 %. Owing to lower R _T, terrestrial anuran species required lower cardiac power outputs when moving fluid through their vasculature compared to aquatic species. These results indicate that physical characteristics of the vasculature can account for interspecific differences in cardiovascular physiology and suggest a co-evolution of cardiac and vascular anatomy among amphibians.