Net cardiac shunts in anuran amphibians: physiology or physics?

Abstract

Amphibians have a single ventricle and common conus arteriosus that produces an equal pressure to the parallel pulmocutaneous and systemic vascular circuits. The distribution of blood flows between the pulmocutaneous (Q pul ) and systemic (Q sys ) circuits (net cardiac shunt) varies with a number of environmental conditions and behaviours; although autonomic regulation of pulmonary vascular resistance conductance has been emphasized, little attention has been paid to the possible contribution of the passive physical characteristics of the two circuits to pressure changes associated with variation in cardiac output. In this study, we re-analysed three recent studies that recorded net cardiac shunts in the cane toad (Rhinella marina) under a variety of conditions and treatments. In all three studies, Q pul and Q sys were linearly related to cardiac output (Q tot ), but the slope was threefold higher for Q pul compared with Q sys as predicted by relative conductance increases associated with increases in pressure from perfused preparations where autonomic regulation and humoral control were eliminated. Our analysis indicates that the net cardiac shunt in the cane toad is predicted primarily by the physical, rather than physiological, characteristics of the parallel pulmonary and systemic vascular circuits.

Vagal tone regulates cardiac shunts during activity and at low temperatures in the South American rattlesnake, Crotalus durissus

“…Although many mechanisms may influence cardiac shunts-humoral factors (Crossley et al 2000;Galli et al 2005a, b;Joyce and Wang 2014), adrenergic tone (Hicks 1994;Galli et al 2007) and intrinsic differences between systemic and pulmonary vascular distensibilities (Kohl et al 2013;Hillman et al 2014)-left vagotomy completely inverted the shunt patterns observed at 15 °C. This is in accordance with the observations of cholinergic control of R-L shunts in T. scripta .…”

“…For amphibians, it was recently argued that the rise in L-R shunt at high cardiac output (CO) is mainly driven by inherent physical properties of the vascular system, where the high distensibility of the pulmonary circuit allows for increased pulmonary blood flow (Hillman et al 2014;Kohl et al 2013). In reptiles, activity in the vagal innervation of the pulmonary artery can massively reduce G pul and increased vagal tone can mediate large net R-L shunts (Burggren 1987;Luckhardt and Carlson 1921;Milsom et al 1977;Taylor et al 2009).…”

A meta-analysis ofin vivovertebrate cardiac performance: implications for cardiovascular support in the evolution of endothermy

“…Increases in cardiac output also increase conductance by physically dilating resistance vessels, based on the increased volume added to the arterial vasculature. It is likely that the mechanisms matching conductance and flow are probably a combination of both passive physical effects and physiological effects mediated through negative feedback (Hillman et al, 2014). The difference between ectotherms and endotherms in the slope of the conductance-flow relationship can be attributed to the endotherms maintaining a higher pressure (17.1 kPa) compared with ectotherms (3.3 kPa).…”

Analysis of vascular mechanical properties from the yellow anaconda indicates increased elasticity and distensibility of the pulmonary artery during digestion

“…Arterial mechanics have been suggested to influence intracardiac blood shunt patterns in organisms without functional ventricular flow separation (Filogonio et al, 2017a,b;Hillman et al, 2014Hillman et al, , 2017. In short, the equal systolic pressures in the systemic and pulmonary circuits would distend the more distensible vessels, thus delivering a greater portion of the stroke volume to that circuit (Hillman et al, 2014).…”

“…Arterial mechanics have been suggested to influence intracardiac blood shunt patterns in organisms without functional ventricular flow separation (Filogonio et al, 2017a,b;Hillman et al, 2014Hillman et al, , 2017. In short, the equal systolic pressures in the systemic and pulmonary circuits would distend the more distensible vessels, thus delivering a greater portion of the stroke volume to that circuit (Hillman et al, 2014). Given the lack of studies concerning the typical reptilian cardiovascular circuit, in this study we compared the mechanical properties and collagen, elastin and sGAG content of the dorsal aorta and the pulmonary artery from the yellow anaconda, Eunectes notaeus Cope 1862a species without intraventricular separation (Jensen et al, 2014).…”