Does the left aorta provide proton-rich blood to the gut when crocodilians digest a meal?

Abstract

Reptiles have the capacity to differentially perfuse the systemic and pulmonary vascular circuits via autonomic regulation of the heart and the vascular trees. While this aptitude is widely recognized, the role of “shunting” as a homeostatic mechanism to match convective transport with tissue demand remains unknown. In crocodilians, it has been hypothesized that a pulmonary vascular bypass of systemic venous blood, a right-to-left shunt (R-L), serves to deliver CO2-rich blood with protons needed for gastric acid secretion during digestion. This hypothesis is partially based on the unique crocodilian vascular anatomy where a left aorta (LAo) arises from the right ventricle, and appears to preferentially supply the gastrointestinal system, whereas the right aorta emerges from the left ventricle. Recent theoretical considerations imply that a R-L shunt has minuscule effects on PCO2 (Malte et al., 2017), but direct measurements of blood gases in both the right and left aortae or both the right and left atria in fed animals have not been conducted. For this reason, we measured blood parameters including PO2, PCO2, pHe, [HCO3−] in both aortae (right and left) as well as both atria following ingestion of a gavage feed standardized meal (5% body mass). Blood samples were withdrawn at 3, 6, 12, 24, 36 and 48h into the digestive period to encompass the period of maximal gastric acid secretion. At no point did PCO2 or pH differ between the left and right aortae, whereas PO2 was significantly lower in the left aorta at several time points during digestion. Our findings do not support the hypothesis that a R-L shunt serves to deliver CO2 for the gastrointestinal system after feeding in crocodilians.