Gas exchange in frogs and turtles: how ectothermic vertebrates contributed to solving the controversy of pulmonary oxygen secretion

The mechanisms governing pulmonary gas exchange were heavily debated at the start of the 20th century when Christian Bohr provided measurements of lung and blood gases as well as rational arguments in favour of oxygen being secreted actively from the lung gas to the blood within vertebrate lungs. The concept of active transport was studied by August Krogh in his doctoral dissertation on the partitioning of gas exchange in frogs. In later studies, where Marie and August Krogh provided conclusive evidence that pulmonary gas exchange occurs by diffusion and diffusion alone, the turtle lungs provided an important tool to investigate the role of perfusion in pulmonary gas exchange. Here, I review the early Bohr and Krogh studies on pulmonary and cutaneous gas exchange in frogs as well as the experimental studies on gas exchange and its possible autonomic regulation in turtles. The results are discussed within the context of recent studies on the cardiorespiratory physiology of frogs and turtles.