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 P_CO , R-L shunt also reduces arterial O₂ levels and would compromise O₂ delivery during the increased metabolic state of digestion. Conversely, arterial P_CO  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.