The common understanding of a heart is based on the four-chambered model found in mammals. The animal kingdom, however, presents a wide array of cardiovascular designs, with the reptilian heart being a sophisticated example of evolutionary adaptation. The hearts of reptiles are not simpler versions of their mammalian counterparts; they are uniquely structured to support a distinct physiology. These differences allow for circulatory capabilities tuned to challenges like holding their breath for long periods and managing body temperature.
The Three-Chambered Heart Design
Most non-crocodilian reptiles, including lizards, snakes, and turtles, possess a three-chambered heart. This structure consists of two upper chambers, the right and left atria, and a single, larger lower chamber, the ventricle. The right atrium receives deoxygenated blood returning from the body, while the left atrium collects oxygenated blood from the lungs. Both atria then empty their contents into the common ventricle, which pumps blood to the lungs and the rest of the body.
The single ventricle is not a simple space, as it is partially divided by a thick, muscular ridge that functions as a partial septum to direct blood flow. The ventricle is composed of three interconnected subchambers: the cavum pulmonale, cavum arteriosum, and cavum venosum. This internal structure allows the three-chambered heart to function with a high degree of separation between oxygenated and deoxygenated blood.
This arrangement of subchambers and the muscular ridge prevents the complete mixing of blood. During contraction, the movement of the ventricular walls and valve closures channel deoxygenated blood toward the pulmonary artery. Simultaneously, oxygenated blood is directed toward the two aortic arches for systemic circulation. This functional separation allows the three-chambered heart to operate almost like a four-chambered one under normal conditions.
The Functional Advantage of Blood Shunting
The structure of the three-chambered heart enables intracardiac shunting, the ability to redirect blood flow between the pulmonary and systemic circuits. This provides a physiological advantage tied to reptilian behaviors like apnea, or breath-holding. Shunting allows reptiles to adjust their circulation in response to metabolic needs, a level of control not possible in the hearts of mammals.
A right-to-left shunt is when deoxygenated blood is redirected to bypass the lungs and enter the systemic circulation. This is useful for aquatic turtles during long dives. When submerged, the lungs are not in use, so sending blood to them is an inefficient use of energy. By shunting blood away from the pulmonary circuit, the animal conserves energy.
This same right-to-left shunt also plays a part in thermoregulation. When a reptile is basking, shunting allows blood to bypass the cooler lungs, a potential site of heat loss. This helps recirculate warmed blood more rapidly through the body, raising the core body temperature more quickly. Shunting also aids in stabilizing blood oxygen levels during apnea.
The Crocodilian Exception
While the three-chambered heart is the norm for most reptiles, crocodilians—including crocodiles, alligators, and caimans—are a notable exception. They evolved a four-chambered heart, structurally similar to those in birds and mammals. Their hearts feature two atria and two completely separate ventricles, meaning there is no mixing of oxygenated and deoxygenated blood within the heart.
This complete separation of the ventricles would seem to prevent the blood-shunting capabilities seen in other reptiles. The presence of a four-chambered heart in crocodilians is a case of convergent evolution, where a similar trait evolves independently in different lineages. Despite this similarity to mammals, the crocodilian circulatory system retains unique features.
These features allow crocodilians to perform controlled blood shunts through a different mechanism than other reptiles. Even with four fully divided chambers, their circulatory system is engineered to bypass the pulmonary circuit when needed. This demonstrates that the functional need for circulatory control remains, even with a different anatomical solution.
Unique Crocodilian Circulatory Adaptations
The crocodilian heart achieves shunting through two anatomical features outside the heart itself. The first is the Foramen of Panizza, a channel connecting the left and right aortic arches. The right aorta emerges from the left ventricle carrying oxygenated blood, while the left aorta exits the right ventricle with deoxygenated blood. The Foramen of Panizza allows blood to pass between these two arteries.
Working with this foramen is the cog-tooth valve, located at the base of the pulmonary artery. This valve is composed of connective tissue nodules that function like the teeth of a gear.
When a crocodilian is breathing air, blood pressure in the left ventricle is higher than in the right. This causes oxygenated blood to flow from the right aorta into the left aorta via the Foramen of Panizza, ensuring the brain receives highly oxygenated blood.
At the same time, the cog-tooth valve remains open. This allows deoxygenated blood from the right ventricle to flow normally to the lungs.
During a dive, the situation changes. The crocodilian holds its breath, and pressure builds in the pulmonary circuit. This increased resistance causes the cog-tooth valve to engage, restricting or blocking blood flow to the pulmonary artery.
As pressure in the right ventricle rises to exceed that of the systemic circuit, deoxygenated blood is forced out through the left aorta, bypassing the lungs. This system allows crocodilians to achieve a controlled right-to-left shunt with a different anatomical toolkit than other reptiles.