Vascular distensibilities have minor effects on intracardiac shunt patterns in reptiles

The different vascular distensibilities of systemic and pulmonary circuits were recently proposed as an important mechanism defining the direction of cardiac shunt and the distribution of cardiac output in vertebrates with undivided cardiac ventricles. In short, the more distensible pulmonary vascular bed was proposed to accommodate a larger portion of the blood ejected from the heart when cardiac output increases. To evaluate this hypothesis, we performed a meta-analysis based on fourteen previously published studies in two species of reptiles (the freshwater turtle, Trachemys scripta, and the South American rattlesnake, Crotalus durissus) to describe how cardiac shunt patterns change when cardiac output increases. We found no general linear relation between total blood flow and systemic or pulmonary blood flows. Thus, cardiac output alterations cannot be used as a reliable reference for prediction of shunt patterns in both T. scripta and C. durissus. Hence, differential distensibilities appear to be of minor importance for the determination of cardiac shunt patterns in the species analyzed. We speculate that the lack of correlation in these phylogenetically distant species may indicate that this is a common trend within non-crocodilian reptiles.