The Functional Significance of the Reptilian Heart: New Insights into an Old Question

Abstract

The vertebrate heart has undergone an extensive evolutionary remodeling from a "piscine cardiac design" with a single atrium and a single ventricle to a double circulatory system in air-breathing vertebrates with two separate atria and a partial or complete division of the ventricle. The anatomical division of the ventricle and the complete separation of the systemic and pulmonary circulations seem to have evolved independently on at least two occasions within terrestrial air-breathing vertebrates. This independent evolution is evident from Fig. 9.1 , which depicts the phylogenetic relationship amongst amniotes, where a divided ventricle appeared within the synapsid lineage that gave rise to mammals, and within archosaurs that gave rise to crocodilians and birds. It is also possible that the four-chambered heart arose separately within extant crocodilians and birds, respectively, but this scenario would require that the ventricular division had occurred at least three times independently.The divided ventricle provides the ability to increase systemic arterial blood pressures, while keeping low blood pressures in the pulmonary circulation. This low pulmonary pressure allows for a thinner air-blood barrier within the lungs, which contributes to a larger diffusion capacity and hence more ef fi cient gas exchange. A division of the ventricle also avoids cardiac shunts, i.e. the mixture of oxygen-rich

Right-to-left shunt has modest effects on CO2 delivery to the gut during digestion, but compromises oxygen delivery

“…In support for elevated (systemic) arterial partial pressure of CO 2 (P CO2 ) governing acid secretion, Farmer et al (2008) reported slower digestion after surgical removal of the left aorta in alligators. However, a number of other studies show that growth is not affected by similar procedures (Eme et al, 2009(Eme et al, , 2010, and it is possible that the slower digestion stems from reduced perfusion of the gastrointestinal organs after occlusion of the left aortic arch (Hicks and Wang, 2012).…”

“…However, whilst much is known about the anatomical basis for central vascular shunts and their autonomic regulation, the functional role of bypassing one or the other circulation remains as mysterious as it is debated (Hicks and Wang, 2012). Thus, it remains uncertain as to whether this cardiovascular design is an exquisite adaptation to low ectothermic metabolism and intermittent pulmonary ventilation, or merely an atavistic relict with no particular functional benefits (Hicks and Wang, 2012).…”

“…the direct recirculation of systemic venous blood into the arterial systemic circulation) decrease whenever oxygen demands are elevated (Hicks and Wang, 2012). However, in crocodilians, an elevated oxygen consumption associated with digestion may be an exception.…”

DIVING IN A WARMING WORLD: Thermal constraints on the diving capacity of estuarine crocodiles (Crocodylus porosus)

“…This redirection of blood flow has been hypothesised to be extend aerobic dive limits by conserving lung oxygen stores, sequestering carbon dioxide away from the lungs and inducing hypometabolism (White, 1969;Hicks and Wang, 2004). However, several studies have rejected this hypothesis, with the view that an adaptive function of the PBS may not exist (Hicks, 2002;Eme et al, 2009;Hicks and Wang, 2012). The initiation of the 'dive response' appears to be context specific in some species (Gaunt and Gans, 1969;Noren et al, 2012); with crocodylians, for example, markedly reducing heart rate (65 ± 6% reduction) during predator avoidance dives (i.e.…”

“…Crocodylians experience a pulmonary by-pass shunt during predator avoidance dives, which is hypothesised to induce metabolic depression and facilitate prolonged dives (reviewed in Grigg and Kirshner, 2015;c.f. Farmer et al, 2008;Eme et al, 2009;Hicks and Wang, 2012).…”