Evolution of the vertebrate cardio-pulmonary system

Vertebrate lungs have long been thought to have evolved in fishes largely a s an adaptation for life in hypoxic water. This view overlooks the possibil ity 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 ve rtebrates is avascular and obtains oxygen from luminal blood. Because oxyge n-rich pulmonary blood mixes with oxygen-poor systemic blood before enterin g the heart of air-breathing fishes, lung ventilation may supply the myocar dium with oxygen and expand aerobic exercise capabilities. Although sustain ed exercise in tetrapods is facilitated by septation of the heart and the f ormation of a dual pressure system, a divided cardiopulmonary system may co nflict with myocardial oxygenation because the right side of the heart is i solated from pulmonary oxygen. This may have contributed to the evolution o f the coronary circulation.