Cardiovascular function in ectotherm sauropsids

Although the anatomy of the heart and its outflow tract differs between crocodiles and all other extant ectotherm sauropsids (i.e. Chelonia, Rhynchocephalia, and Squamata), they share one unique feature: All ectotherm sauropsids are able to bypass the pulmonary circulation and to redirect blood volumes returning from the systemic circulation directly back into the systemic circulation. Lepidosauria and Testudines use the interconnected three chambers of their ventricle for pulmonary bypass, and they are also able to bypass the systemic circulation. All ectotherm sauropsids possess two aortae. In crocodiles, they emerge from different ventricles, which is the mechanism that enables a pulmonary bypass. Their aortae are interconnected by two structures: The Foramen Panizzae and the aortic anastomosis. They enable exchange of blood volumes between the two aortae carrying blood from the two ventricles. This thesis evaluates the functional significance of bypass and blood flow through the Foramen Panizzae and the anastomosis from two sides: First, it examines the morphology of bypass related structures in crocodiles and snakes using a variety of histological and bioimaging methods. These methods are also compared regarding their strengths and weaknesses for the examination of the structures of interest in. Second, a computational multi-compartment model was developed to simulate blood volume distribution in dependence from pulmonary bypass and the blood flow through the Foramen Panizzae and the aortic anastomosis in the crocodilian body. Additionally, I examined the impact of these parameters on physiological measurements that are often used in physiological experiments on the cardiovascular function in ectotherm sauropsids. Apart from supporting knowledge on some already examined anatomical details, the present contribution adds information on so far unexamined species and structures, amongst them a description of the cardiac cartilage clasp in Crocodylus niloticus while the function of bypass in snakes remains unclear. The data from histology and bioimaging contributed to the paramaters chosen for the simulation model. The results of the simulation indicate that – apart from its known impact on supplying the lungs and digestive tract – pulmonary bypass reduces blood supply of the anterior and posterior body regions of crocodiles. Blood flow through Foramen Panizzae and aortic anastomosis is from the right into the left aorta without pulmonary bypass and inverts at its onset. As a consequence of vascular arrangement, decreasing blood flow into the pulmonary circulation during pulmonary bypass decreases left-ventricular return and consequently total cardiac stroke volume, with zero left-ventricular return at complete bypass. I conclude that pulmonary bypass must be incomplete or of limited duration. The simulation model shows that blood flows balance within a few heart beats after a change in pulmonary bypass, or Foramen Panizzae/ aortic anastomosis flow. The results support the idea that pulmonary bypass may improve digestive function by supporting gastric acid secretion and buffering the postprandial alkaline tide rather than being an adaptation to extended diving periods in crocodiles. The disadvantageous effects of pulmonary bypass on oxygen supply to the digestive tract may be eliminated by the so far undiscussed course of branches of the right aorta which additionally supply the digestive tract with oxygen-rich blood. Pulmonary bypass might also contribute to thermoregulation.