There are large differences in the morphology of the vertebrate heart; from the fish heart with its single atrium and single ventricle to the crocodilian, bird and mammalian hearts with two fully separated atria and ventricles (Van Mierop & Kutsche, 1985). It is only in crocodilians, birds and mammals where the heart has a complete interventricular septum, that a full intracardiac separation of blood pressure and flow in the systemic and pulmonary circulations can occur. In birds and mammals the left ventricle gives rise to the aorta supplying the body with oxygenated blood and the pulmonary arteries arise from the right ventricle carrying deoxygenated blood to the lungs. In these two animal groups no intra- or extra-cardiac mixing of blood (shunting) is possible, and no shunting of blood between the pulmonary and systemic circulations occurs in healthy adults. The crocodilians are unique in comparison to other reptiles and also to birds and mammals. In comparison to other reptiles (snakes, lizards and turtles) the crocodilian heart is unique in that the ventricle is fully divided into a left and a right ventricle whereas the non-crocodilian reptile heart is subdivided into three intraventricular compartments that are interconnected (no morphological subdivision of the ventricle) allowing intracardiac mixing of oxygenated and deoxygenated blood. The crocodilian heart is also unique compared to the bird and mammalian heart in that shunting of blood away from the pulmonary circulation is still possible as the right ventricle gives rise not only to the pulmonary arteries but also to the left aorta (Fig. 1). This allows deoxygenated blood from the right ventricle to bypass the lungs and to be recirculated into the systemic circulation (pulmonary-to-systemic shunt). Apart from the 'extra' left aorta from the right ventricle, three other morphological features of the crocodilian cardiovascular system have been the focus of discussion over recent years. (1) The foramen of Panizza; an opening between the right and left aorta situated in the common wall of the left and right aorta (Fig. 1A). (2) The subpulmonary conus situated in the pulmonary outflow tract of the right ventricle (Fig. 1B). (3) The aortic anastomosis that connects the two aortic arches just posterior to the heart (Fig. 1C; van Mierop & Kutsche, 1985). The subpulmonary conus contains connective tissue nodules protruding into the outflow tract that acts as an 'extra' and unique valve mechanism (Fig. 1B). These three areas have received much attention from comparative physiologists interested in the function of the crocodilian/reptile circulation, and the evolution of the cardiovascular system and its regulation. There is increasing evidence for a close regulation of the three unique structures in the crocodilian cardiovascular system and their importance for the normal function of the crocodilian circulation. This short overview will give a summary of the latest findings.

Experimental Physiology (2001) 86.6, 785-789.