Functional role of cardiac shunts in reptiles

Cardiac shunting normally occurs in chelonians (turtles), squamates (lizards and snakes) and crocodilians (alligators, caimans, crocodiles). The magnitude of these shunts is affected by the ventilatory state and provides these animals with a flexibility in blood flow that cannot be achieved in mammals or birds. The differential distribution of blood flow into the pulmonary and systemic circulations has prompted physiologists to postulate the functional significance of such a circulatory design. Most authors agree that cardiac shunts are not atavistic relics, but represent an important cardiovascular adaptation in extant reptiles. Nevertheless, while these hypotheses may be intuitively appealing, experimental evidence attesting the advantageous role of cardiac shunts in pulmonary gas exchange is lacking. The purpose of this paper is to review, in detail, the various hypotheses that explain the functional significance of cardiac shunts in pulmonary gas exchange in intermittently breathing reptiles.