Physiology: An actively controlled heart valve

Vertebrate hearts typically have cardiac valves that are thin and leaf-like and which work passively, allowing blood to move forward during systole and preventing it from flowing back during diastole. Crocodilian hearts have nodules of connective tissue, resembling opposing knuckles, or cog-teeth, in the subpulmonary conus just proximal to the pulmonary valves. Here we show that these cog-teeth act in the estuarine crocodile Crocodylus porosus (Fig. 1<figr rid="f1"></figr>) as a valve that regulates the flow of blood between the lungs and the systemic circulation in response to a β-adrenergic mechanism. To our knowledge, this is the first report of an actively controlled intra-cardiac valve in a vertebrate.