Structure and function of the hearts of lizards and snakes

Abstract

With approximately 7000 species, snakes and lizards, collectively known as squamates, are by far the most species-rich group of reptiles. It was from reptile-like ancestors that mammals and birds evolved and squamates can be viewed as phylogenetically positioned between them and fishes. Hence, their hearts have been studied for more than a century yielding insights into the group itself and into the independent evolution of the fully divided four-chambered hearts of mammals and birds. Structurally the heart is complex and debates persist on rudimentary issues such as identifying structures critical to understanding ventricle function. In seeking to resolve these controversies we have generated three-dimensional (3D) models in portable digital format (pdf) of the anaconda and anole lizard hearts ('typical' squamate hearts) and the uniquely specialized python heart with comprehensive annotations of structures and cavities. We review the anatomy and physiology of squamate hearts in general and emphasize the unique features of pythonid and varanid lizard hearts that endow them with mammal-like blood pressures. Excluding pythons and varanid lizards it is concluded that the squamate heart has a highly consistent design including a disproportionately large right side (systemic venous) probably due to prevailing pulmonary bypass (intraventricular shunting). Unfortunately, investigations on rudimentary features are sparse. We therefore point out gaps in our knowledge, such as the size and functional importance of the coronary vasculature and of the first cardiac chamber, the sinus venosus, and highlight areas with implications for vertebrate cardiac evolution.

In situcardiac perfusion reveals interspecific variation of intraventricular flow separation in reptiles

“…By contrast, mammals, birds and crocodilians have two anatomically distinct atria and ventricles, wherein the right atrium exclusively fills the right ventricle with oxygen-poor systemic venous blood, and the left atrium supplies the left ventricle with oxygenated blood returning from the lungs. Anatomically 'intermediate' are the hearts of amphibians and non-crocodilian reptiles (turtles, lizards and snakes), where oxygen-poor and oxygen-rich blood from the right and left atria converge within a single ventricle (Hicks, 2002;Jensen et al, 2014). Reptiles thus present an ideal paradigm to investigate the evolution of ventricular complexity coupled to the double circulatory system.…”

“…Blood returning from the lungs arrives at the cavum arteriosum from where it is pumped into the left (LAo) and right (RAo) aortic arches via the cavum venosum (Hicks, 2002). The cavum arteriosum and cavum venosum are partially separated from the cavum pulmonale by a myocardial structure known as the muscular ridge (MR) (van Mierop and Kutsche, 1985;Hicks, 2002;Jensen et al, 2014). The MR can effectively separate pulmonary and systemic venous blood within the ventricle, as illustrated by numerous classical studies demonstrating that blood emanating from the systemic arches and pulmonary artery differ substantially in their oxygen concentrations in turtles (Steggerda and Essex, 1957), snakes (White, 1959) and lizards (Foxon et al, 1956;White, 1959).…”

“…Despite the intraventricular compartmentalisation, the single ventricle presents the opportunity for the mixing of oxygen-rich and oxygen-poor blood (intracardiac shunting) (Hicks, 2002;Hicks and Wang, 2012;Jensen et al, 2014). Intracardiac shunting may be described as right to left (R-L), i.e.…”

The Epicardium in Ventricular Septation During Evolution and Development

“…We prefer to address this structure as 'folding septum'. The presence and extent of the vertical septum (homologous to inlet septum) differs among reptiles, being virtually absent in turtles and most non-crocodilian reptiles, but more prominent in varanids and pythonidae [11]. In the python myocardial apical trabeculations traverse the ventricle to partly separate the cavum pulmonale from the cavum arteriosum.…”

Electrical and Contractile Properties of the Heart Ventricle in Response to Ambient Temperature Changes in Frog Rana temporaria

“…Therefore, ectothermic animals can be used to understand mechanisms controlling the electrical and contractile functions of the myocardium in response to changes in environmental temperature. (5)(6)(7) At present there are only a few studies of the electrophysiological processes that occur in a frog's heart ventricle under changes in ambient temperature. (8,9) It has been shown that when the body temperature in the frogs is decreased to 10ºС, then HR is diminished and the durations of QRS and ST-T complexes of ECG are lengthened.…”