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

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 (R–L) shunt – serves to deliver CO-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 would have minuscule effects on P, 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 P, P, pH and [HCO] in the right and left aortae and atria following ingestion of a gavage-fed standardized meal (5% body mass). Blood samples were taken at 3, 6, 12, 24, 36 and 48 h into the digestive period to encompass the period of maximal gastric acid secretion. At no point did P or pH differ between the left and right aortae, whereas P 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 CO for the gastrointestinal system after feeding in crocodilians.