Hunting & Shunting in Reptiles

Left-to Right Shunt and Myocardial Oxygenation:

All ectothermic reptiles have the ability to shunt blood. In chelonians (turtles and tortoises) and lepidosaurs (tuatara, lizards and snakes) the cardiac ventricle is partially divided by two septa, forming three chambers within the heart. The lack of complete septation enables intracardiac shunting. Because these animals have a great deal of neurological control over patterns of blood flow, it is plausible that shunting has conferred selective advantages, although the benefits of shunting remain poorly understood. The mechanism of shunting depends on the resistance in the pulmonary artery and the left and right aortas, and depends on the state of the heart at the after ejection. A left-to-right intracardiac shunt occurs when oxygen-rich blood, which has entered the left atrium and moved into the cavum arteriosum in the left side of the ventricle, is pumped into the pulmonary artery, to be recirculated to the lung. This shunt may function to provide the heart with oxygen (Farmer and Hicks, 2002). As in fishes, the hearts of these animals are composed primarily of spongy myocardium lacking significant support from a coronary circulation. The need for oxygen of the heart is greatest during periods of heavy work, such as during exercise. Therefore, if the shunt serves myocardial oxygen it should occur when the animals are active, which it does.  The the electrocardiogram is one of the most sensitive indicators of myocardial hypoxia, and we recorded the ECG, as well as direct measurements of cardiac power, to assess the potential benefits of this shunt for myocardial oxygenation during periods of exercise Farmer and Hicks, 2002.

Right-to-Left Shunt and Digestion:

Non-avian reptiles can also direct oxygen-poor blood that is returning to the heart from the body back to the body, bypassing the lung, the right-to-left shunt. In crocodilians the ventricle is completely separated into right and left sides, as it is in mammals and birds. This prevents an intracardiac shunt, but because the left aorta originates from the right side of the cardiac ventricle crocodilians can still right-to-left shunt by closing a cog-tooth valve in the pulmonary artery and ejecting right ventricular blood into the left aorta. We have tested the hypothesis that sending right ventricular blood, which is acidic and hypercapnic, to the stomach increases rates of gastric acid secretion, thereby facilitating digestion of food, by surgically blocking the flow from the right ventricle into the left aorta and measuring rates of gastric acid secretion and digestion (Farmer et al. 2008, Farmer, 2011).


Schematic (ventral view) of blood flow in the heart of a turtle during diastole (filling phase) & photograph (cranial view) illustrating inflow and outflow tracks

Oxygen-rich blood (red) returning to the heart from the lungs enters the left atrium and the left side of the ventricle, the cavum arteriosum. Oxygen-poor blood (blue) that is flowing to the heart from the rest of the body enters the right atrium and then flows into a central ventricular chamber, the cavum venosium. This blood mixes with the residual blood in the chamber, which is oxygen-rich, and then the admixture flows into the right side of the heart, the cavum pulmonale. Without this mixing, the right side of the heart would contain only the oxygen-depleted blood. After Farmer and Hicks, 2002


Schematic (ventral view) of blood flow in the heart of a turtle during Systole (contraction and ejection) & photograph (cranial view) showing inflow and outflow tracks

The admixture in the cavum pulmonale is ejected directly into the pulmonary artery, sending the partially enriched blood back to the lung. The oxygen-rich blood in the cavum arteriosum must travel through the cavum venosum before being ejected into the left and right aortas. Thus the residual blood in this chamber at the end of systole is oxygen-rich and is the source of the left-to-right shunted blood on the next diastolic phase. After Farmer and Hicks, 2002


Cardiac anatomy of alligator heart and great vessels, (ventral view)

A) Schematic (after Greenfield and Morrow, 1961) of the heart showing inflow into the ventricle from the right and left atria and the outflow of the left aorta and pulmonary artery. Inset shows the connection between the left and right aortas (the foramen of Panizza). (B) Photograph of this anatomy (ventral view). The ability of the animals to eject oxygen-poor blood into the LAo is created by partial closure of a cog-tooth valve in the pulmonary artery, which results in a pressure rise large enough in the right ventricle to cause blood to be ejected into the left aorta. Shunting occurs when animals are in a parasympathetic state, when in a state of excitement there is no shunting and the cardiovascular system works that that of a mammal or bird.


This outflow tract was blocked by surgical occlusion at the points marked by yellow dots (from Farmer et al. 2008). cc=common carotid, LPA=left pulmonary artery, PA=common pulmonary arterial trunk, LAo=left aorta, RAo=right aorta, RAt=right atrium, LAt= left atrium, RA-V aperture=right atrial-ventricular aperture, RPA=p.





Anatomy and shunting pattern in Alligators

A) Schematic of cardiac anatomy (ventral view) without an intracardiac shunt. Oxygen-rich blood (red) is ejected into the right aorta and then flows into the left aorta through the forament of Panizza. B) Schematic of flow during a shunt, oxygen-poor blood flows into the left aorta from the right ventricle. C) Angiogram with radiopaque material injected into the right ventricle along with acetylcholine to test the effectiveness of surgical occlusion. C) Angiogram with radiopaque material flowing into the left aorta from the right ventricle. E) Pattern of flow in the left aorta without a shunt. F) Flow in the left aorta during shunting. (from Farmer et al. 2008).

Digestive anatomy of the the American alligator, ventral view

C) Schematic (after Reese 1915) and (D) photo (Farmer et al. 2008) of the left and right aortas and the arrangement of the blood vessels feeding the gastroinstinal system. When crocodilians right-to-left shunt, blood that is rich in acid and carbon dioxide (blue) flows through the left aorta to irrigate the stomach, spleen, pancreas, and small intestine. Blood to the large intestine and hindlegs and tail is an admixture from left and right aorta. A=anastomosis between left and right aortas, C=celiac, DAo=dorsal aorta, LAo=left aorta, RAo=right aorta, S=stomach.

Metabolic pathways that use carbon dioxide for synthesis or that are affected by pH

A number of synthetic pathways make use of carbon dioxide or bicarbonate. These include the production of gastric acid, dudoenal base, the synthesis of fatty acids, the formation of purine rings important in the production of ATP, DNA, RNA etc. Levels of acid and carbon dioxide also affect synthetic processes in the bone.  After Farmer 2011


Farmer, CG 2011. On the Evolution of Vascular Patterns of Tetrapods. Journal of Morphology 272(11):1325-1341. PDF


Farmer, CG., TJ Uriona, M. Steenblik, D. Olsen, K. Sanders.  2008. The right-to-left shunt of crocodilians serves digestion. Physiological and Biochemical Zoology 81(2):125–137, (cover). PDF


Farmer, CG. and J.W. Hicks. 2002. The intracardiac shunt as a source of myocardial oxygen in a turtle, Trachemys scripta.& Integrative and Comparative Biology 42(2):208-215. PDF