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 blooda right-to-left (R-L) shuntserves to deliver CO 2 -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 CO2 , 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 O2 , P CO2 , pH e and [HCO 3 − ] 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 CO2 or pH differ between the left and right aortae, whereas P O2 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 2 for the gastrointestinal system after feeding in crocodilians.

New insights into the allosteric effects of CO2 and bicarbonate on crocodilian hemoglobin

“…However, whether CO 2 and bicarbonate content within RBCs also increases during digestion remains to be investigated. In addition, the possibility of right-to-left shunts in the unique crocodilian heart adds further complexity in these situations through their influence O 2 and CO 2 /bicarbonate concentrations in the blood (Conner et al, 2019;Malte et al, 2016).…”

Cardiovascular shunting in vertebrates: a practical integration of competing hypotheses

“…However, the proposal that the R–L shunt facilitates digestion is problematic because the shunted blood is low in oxygen and thus lower oxygen delivery would coincide with an elevated oxygen consumption due to the specific dynamic action (SDA) response (Busk et al ., ). Modelling of gas exchange shows that R–L shunts exert rather small effects on P CO 2 because CO 2 has such high solubility in blood (Malte et al ., ), and these predictions were verified by experimental data (Conner et al ., ). Consequently, it seems very doubtful that a R–L shunt could serve a role in stimulating gastric acid secretion.…”

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 with many activities including diving or pH-related postprandial metabolism. The functions [19,61] also require the contraction of specialized muscle nodules (dubbed "cogteeth" or "cogwheels" [62,63]) in the right ventricular outflow to increase the right ventricular blood pressure over a threshold to allow the mechanism of opening (and closing) of the foramen by the guarding septal valve leaflet in the systemic aorta [64].…”