Cardiovascular shunting in vertebrates: a practical integration of competing hypotheses

Abstract

This review explores the long‐standing question: ‘Why do cardiovascular shunts occur?’ An historical perspective is provided on previous research into cardiac shunts in vertebrates that continues to shape current views. Cardiac shunts and when they occur is then described for vertebrates. Nearly 20 different functional reasons have been proposed as specific causes of shunts, ranging from energy conservation to improved gas exchange, and including a plethora of functions related to thermoregulation, digestion and haemodynamics. It has even been suggested that shunts are merely an evolutionary or developmental relic. Having considered the various hypotheses involving cardiovascular shunting in vertebrates, this review then takes a non‐traditional approach. Rather than attempting to identify the single ‘correct’ reason for the occurrence of shunts, we advance a more holistic, integrative approach that embraces multiple, non‐exclusive suites of proposed causes for shunts, and indicates how these varied functions might at least co‐exist, if not actually support each other as shunts serve multiple, concurrent physiological functions. It is argued that deposing the ‘monolithic’ view of shunting leads to a more nuanced view of vertebrate cardiovascular systems. This review concludes by suggesting new paradigms for testing the function(s) of shunts, including experimentally placing organ systems into conflict in terms of their perfusion needs, reducing sources of variation in physiological experiments, measuring possible compensatory responses to shunt ablation, moving experiments from the laboratory to the field, and using cladistics‐related approaches in the choice of experimental animals.

A Baseline Model For Estimating the Risk of Gas Embolism in Sea Turtles During Routine Dives

“…Another important difference between marine mammals and sea turtles is that sea turtles have muscular sphincters within their pulmonary arteries and a partially compartmentalized ventricle that allow for central intracardiac shunting ( García-Párraga et al, 2018a ; Burggren et al, 2020 ). These features could allow a complete shunt and cessation of gas exchange as the animal begins the dive.…”

“…In this model, we assumed that the pulmonary shunt develops due to the passive compression of the terminal air spaces and the structural properties of the conducting airways and faveoli ( Bostrom et al, 2008 ; Fahlman et al, 2009 ). While this is not necessarily the case in reptiles, the exact factors influencing the cardiac shunt in sea turtles are not well-established ( Burggren et al, 2020 ). Without having a solid understanding of the functioning of the cardiac shunt, we adjusted the model so that the arterial PN 2 reflected those of forced-diving sea turtles in Berkson (1967) .…”

“…It should also be noted that sea turtles have the capacity to perform a right-to-left intracardiac shunt. As the role of this cardiac shunt in regulating N 2 is unknown in aquatic reptiles ( Burggren et al, 2020 ), we therefore adjusted the parameters for the modeled pulmonary shunt, so that it mirrored the arterial PN 2 levels that previously reported for a sea turtles during forced submergence ( Berkson, 1967 ).…”

Ventricular Septation and Outflow Tract Development in Crocodilians Result in Two Aortas With Bicuspid Semilunar Valves

“…The attributed functions of the FOP have been associated to many activities including diving or pH-related postprandial metabolism. The functions [18,61] require also the contraction of specialized muscle nodules (dubbed "cogteeth" or "cogwheel" [62,63]) in the right ventricular outflow to increase right ventricular blood pressure over a threshold to allow the mechanism of opening (and closure) of the foramen by the guarding septal valve leaflet in the systemic aorta [64]. Intriguingly, the cartilage prongs discussed above are spatially associated with the FOP and the ostium of the systemic aorta, but less with that of the visceral aorta.…”

“…both aortas. After separation and remodeling of the outflow tract this shunt, known as the Foramen of Panizza, (first described by the Italian anatomist Bartolomeo Panizza, 1785-1867 [14,15] connects the roots of the left and right aorta that in specific circumstances may convey blood from the right ventricle to the main circulation [16][17][18] and sometimes vice versa.…”