A comparative study of central cardiovascular dynamics in vertebrates

The dynamic properties of blood flow through the heart and arterial systems have been examined in representative species of fish, amphibia, birds and mammals. In the amphibian examined, the bullfrog Rana catesbeiana. the conus arteriosis was found to perform no active valving function and hence was not responsible for shunting blood to, or away from, the lungs in response to lung ventilation or apnoea. Conus volume changes generated a small fraction of cardiac stroke volume although a low impedance of the pulmocutaneous vasculature resulted in a preferential distribution of this blood to the gas exchanger circulations. The pressure pulse took a negligible fraction of the cardiac cycle to traverse the arterial tree and peripherally recorded pressures were similar in profile to central pressures, these results indicating that wave transmission effects were small. Impedance analysis of pressure and flow data suggested that a two element lumped parameter (windkessel) model accurately describes arterial pressure-flow relationships in the bullfrog. Arterial haemodynamics in the cod, Gadus morhua was examined in terms of a hydraulic model of the 'in series' gill and systemic circulations. Results indicate that the dorsal aorta is not a rigid conduit and the compliance of this vessel has a marked effect on the pulsatility of blood flow through the gills. in the duck. Anas platyrhynchos pressure and flow profiles have been mapped throughout the central circulation. Mean systemic arterial pressure (143 ± 2 mm Hg) and cardiac output (219 ± 7 ml/min per kg) were high compared with mammals of similar size although pulmonary pressures were not high, perhaps because of the unique structure of the avian lung. 75% of total systemic flow was distributed to wing, flight muscles and head by the brachiocephalic arteries. Unlike the situation in the bullfrog wave transmission phenomena had a pronounced effect on arterial pressure and flow signals and impedance data were characterised by features commonly ascribed to the effects of wave reflection. Therefore it is concluded that the windkessel is not a realistic model of the avian arterial system. Pressures generated in both ventricles of the rabbit heart were influenced by contraction of the opposite ventricle although the influence of right ventricular contraction on left ventricular pressure was negligible during normal cardiac function and only became marked when the right ventricular volumes were large or left ventricular volumes were small. Comparison of the effects of vasomotion and artificially induced discrete reflections confirmed that pulse wave transmission effects in mammalian arteries are dominated by reflections from the arteriolar beds. Although studies of a hydraulic model confirmed the viability of transmission line theories on the effects of spatial variations in arterial wall elasticity, close examination suggests that this 'elastic taper' does not have a dominant effect on wave propagation.Science, Faculty ofZoology, Department ofGraduat