The functional anatomy of the crocodilian heart

Abstract: Crocodiles are ectothermic, intermittent breathing reptiles with low metabolic rates. Additionally, they are diving animals that can stay submerged for long periods, which poses special demands on their cardiovascular system. The crocodilian heart is four-chambered with completely separated ventricles, making it unique among other reptiles. It has special anatomical structures that give it the capability to shunt blood away from the lungs, which results in mixing of deoxygenated and oxygenated blood. The purpose of this study was to morphologically describe the heart and review its functions and the significance of its circulatory patterns. Dissection of two crocodilian hearts was performed for morphological characterization of the heart, including the special features that contribute to the shunting: the left aorta, the foramen of Panizza and the cog-teeth-like valves. Obstruction of the pulmonary outflow tract by the cog-teeth-like valves decreases pulmonary blood flow and generates an increased right ventricular pressure, diverting venous blood into the ‘extra’ left aorta and the systemic circulation. Thus, during shunting conditions, venous blood from the left aorta and arterial blood from the right aorta is mixed. The functional significance of this shunting pattern has been extensively discussed and several hypotheses have been proposed. A recent study showed that the absence of the right-to-left shunt does not affect the diving physiology of crocodiles negatively, but it did result in cardiac hypertrophy, indicating that the shunt might have a significance for the circulation and physiology of extant crocodiles.