Evolution of the vertebrate cardio-pulmonary system.

Vertebrate lungs have long been thought to have evolved in fishes largely as an adaptation for life in hypoxic water. This view overlooks the possibility that lungs may have functioned to supply the heart with oxygen and may continue to serve this function in extant fishes. The myocardium of most vertebrates is avascular and obtains oxygen from luminal blood. Because oxygen-rich pulmonary blood mixes with oxygen-poor systemic blood before entering the heart of air-breathing fishes, lung ventilation may supply the myocardium with oxygen and expand aerobic exercise capabilities. Although sustained exercise in tetrapods is facilitated by septation of the heart and the formation of a dual pressure system, a divided cardio-pulmonary system may conflict with myocardial oxygenation because the right side of the heart is isolated from pulmonary oxygen. This may have contributed to the evolution of the coronary circulation.