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

By virtue of their cardiovascular anatomy, reptiles and amphibians can shunt blood away from the pulmonary or systemic circuits, but the functional role of this characteristic trait remains unclear. It has been suggested that right-to-left (R-L) shunt (recirculation of systemic blood within the body) fuels the gastric mucosa with acidified and CO2-rich blood to facilitate gastric acid secretion during digestion. However, in addition to elevating PCO2 , R-L shunt also reduces arterial O2 levels and would compromise O2 delivery during the increased metabolic state of digestion. Conversely, arterial PCO2  can also be elevated by lowering ventilation relative to metabolism (i.e. reducing the air convection requirement, ACR). Based on a mathematical analysis of the relative roles of ACR and R-L shunt on O2 and CO2 levels, we predict that ventilatory modifications are much more effective for gastric CO2 supply with only modest effects on O2 delivery. Conversely, elevating CO2 levels by means of R-L shunt would come at a cost of significant reductions in O2 levels. The different effects of altering ACR and R-L shunt on O2 and CO2 levels are explained by the differences in the effective blood capacitance coefficients.