Form and function in reptilian circulations

Abstract

Consistent with the great variation in their circulatory morphology, there are distinct variations in the cardiovascular physiology of extant reptiles. The chelonian and squamate reptiles have a complexly structured heart that includes three partially separated ventricular cava. In most species (under most conditions), the ventricle acts as a single pressure pump perfusing both the pulmonary and systemic circuits. However, the varanid lizards provide a striking exception. Subtle evolutionary changes in cardiac morphology allow the ventricle of the varanid lizard to divide functionally during systole into a low pressure, pulmonary pump and a high pressure, systemic pump. The crocodilians represent yet another anatomical and physiological pattern. The ventricle is completely divided into left and right chambers as in homeotherms, but the systemic and pulmonary circuits may still communicate through the left aorta that arises from the right ventricle.A fundamental feature of all reptilian circulations is the ability to regulate the distribution of cardiac output between systemic and pulmonary circuits via central vascular blood shunts.Regardless of species, mechanisms for regulating intracardiac shunting involve changes in the balance between peripheral resistance of the pulmonary and systemic circulations, and adjustments in cardiac performance per se. Several hypotheses are presented that suggest selective advantages for central vascular shunting in intermittent breathing reptiles with variable body temperature and metabolic rate. © 1987 by the American Society of Zoologists.