The Right‐to‐Left Shunt of Crocodilians Serves Digestion

Abstract

All amniotes except birds and mammals have the ability to shunt blood past the lungs, but the physiological function of this ability is poorly understood. We studied the role of the shunt in digestion in juvenile American alligators in the following ways. First, we characterized the shunt in fasting and postprandial animals and found that blood was shunted past the lungs during digestion. Second, we disabled the shunt by surgically sealing the left aortic orifice in one group of animals, and we performed a sham surgery in another. We then compared postprandial rates of gastric acid secretion at body temperatures of 19 degrees and 27 degrees C and rates of digestion of bone at 27 degrees C. Twelve hours after eating, maximal rates of gastric acid secretion when measured at 19 degrees and 27 degrees C were significantly less in the disabled group than in sham-operated animals. Twenty-four hours postprandial, a significant decrease was found at 27 degrees C but not at 19 degrees C. For the first half of digestion, dissolution of cortical bone was significantly slower in the disabled animals. These data suggest the right-to-left shunt serves to retain carbon dioxide in the body so that it can be used by the gastrointestinal system. We hypothesize that the foramen of Panizza functions to enrich with oxygen blood that is destined for the gastrointestinal system to power proton pumps and other energy-demanding processes of digestion and that the right-to-left shunt serves to provide carbon dioxide to gastrointestinal organs besides the stomach, such as the pancreas, spleen, upper small intestine, and liver.

Surgical removal of right-to-left cardiac shunt in the American alligator(Alligator mississippiensis) causes ventricular enlargement but does not alter apnoea or metabolism during diving

“…This demonstrates that occluding the LAo only downstream of the great vessel truncus (well downstream of the FoP) does not prevent crocodilian R-L cardiac shunt. Our results are consistent with those of Farmer et al (Farmer et al, 2008) who surgically removed the R-L cardiac shunt in American alligator, and we confirm that LAo occlusion upstream of the FoP is required to completely remove the capacity for R-L cardiac shunt in crocodilians.…”

Surgical removal of right-to-left cardiac shunt in the American alligator(Alligator mississippiensis) causes ventricular enlargement but does not alter apnoea or metabolism during diving

“…This demonstrates that occluding the LAo only downstream of the great vessel truncus (well downstream of the FoP) does not prevent crocodilian R-L cardiac shunt. Our results are consistent with those of Farmer et al (Farmer et al, 2008) who surgically removed the R-L cardiac shunt in American alligator, and we confirm that LAo occlusion upstream of the FoP is required to completely remove the capacity for R-L cardiac shunt in crocodilians.…”

Cardiovascular shunting in vertebrates: a practical integration of competing hypotheses

“…Farmer et al . () provided evidence that alligators with ligated left aortic arches had slower digestion. 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 ., ).…”

Turning crocodilian hearts into bird hearts: growth rates are similar for alligators with and without right-to-left cardiac shunt

“…An overall 9% reduction in mean final body mass and 3-4% reduction in mean final lengths for S-LAo alligators suggests that controlled mixing of central systemic and pulmonary blood is not critical for healthy growth in crocodilians (Table1). The reductions in body mass in the present study and in gastric acid secretion and bone demineralisation found previously (Farmer et al, 2008) may be due to a reduction in blood flow in the coeliac artery and not to a lack of pulmonary bypass shunt, per se. Other studies have shown that forward blood flow from the right ventricle to the LAo can be substantial and is probably a common event (e.g.…”

Surgical removal of right-to-left cardiac shunt in the American alligator(Alligator mississippiensis) causes ventricular enlargement but does not alter apnoea or metabolism during diving

“…This demonstrates that occluding the LAo only downstream of the great vessel truncus (well downstream of the FoP) does not prevent crocodilian R-L cardiac shunt. Our results are consistent with those of Farmer et al (Farmer et al, 2008) who surgically removed the R-L cardiac shunt in American alligator, and we confirm that LAo occlusion upstream of the FoP is required to completely remove the capacity for R-L cardiac shunt in crocodilians.…”

Cardiovascular shunting in vertebrates: a practical integration of competing hypotheses

“…Farmer et al . () provided evidence that alligators with ligated left aortic arches had slower digestion. 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 ., ).…”

Turning crocodilian hearts into bird hearts: growth rates are similar for alligators with and without right-to-left cardiac shunt

“…An overall 9% reduction in mean final body mass and 3-4% reduction in mean final lengths for S-LAo alligators suggests that controlled mixing of central systemic and pulmonary blood is not critical for healthy growth in crocodilians (Table1). The reductions in body mass in the present study and in gastric acid secretion and bone demineralisation found previously (Farmer et al, 2008) may be due to a reduction in blood flow in the coeliac artery and not to a lack of pulmonary bypass shunt, per se. Other studies have shown that forward blood flow from the right ventricle to the LAo can be substantial and is probably a common event (e.g.…”

Surgical removal of right-to-left cardiac shunt in the American alligator(Alligator mississippiensis) causes ventricular enlargement but does not alter apnoea or metabolism during diving

“…This demonstrates that occluding the LAo only downstream of the great vessel truncus (well downstream of the FoP) does not prevent crocodilian R-L cardiac shunt. Our results are consistent with those of Farmer et al (Farmer et al, 2008) who surgically removed the R-L cardiac shunt in American alligator, and we confirm that LAo occlusion upstream of the FoP is required to completely remove the capacity for R-L cardiac shunt in crocodilians.…”

Cardiovascular shunting in vertebrates: a practical integration of competing hypotheses

“…Farmer et al . () provided evidence that alligators with ligated left aortic arches had slower digestion. 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 ., ).…”

Turning crocodilian hearts into bird hearts: growth rates are similar for alligators with and without right-to-left cardiac shunt

“…An overall 9% reduction in mean final body mass and 3-4% reduction in mean final lengths for S-LAo alligators suggests that controlled mixing of central systemic and pulmonary blood is not critical for healthy growth in crocodilians (Table1). The reductions in body mass in the present study and in gastric acid secretion and bone demineralisation found previously (Farmer et al, 2008) may be due to a reduction in blood flow in the coeliac artery and not to a lack of pulmonary bypass shunt, per se. Other studies have shown that forward blood flow from the right ventricle to the LAo can be substantial and is probably a common event (e.g.…”