Circulatory

Thermographic image of a monitor lizard.

Most reptiles have a three-chamber heart consisting of two atria, one variably-partitioned ventricle, and two aorta that go the systemic circulation. The degree of mixing of oxygenated and deoxygenated blood in the three-chamber heart is variable depending on the species and physiological state. Under different conditions, deoxygenated blood can be shunted back to the body or oxygenated blood can be shunted back to the lungs. This variation in blood flow has been hypothesized to allow more effective thermoregulation and longer diving times for aquatic species, but has not been shown to be a fitness advantage.

There are some interesting exceptions to the general physiology. For instance, crocodilians have an anatomically four-chambered heart, but also have two systemic aorta and are therefore capable only of bypassing their pulmonary circulation.^[28] Also, some snake and lizard species (e.g., monitor lizards and pythons) have three-chamber hearts that become functional four-chamber hearts during contraction. This is made possible by a muscular ridge that subdivides the ventricle during ventricular diastole and completely divides it during ventricular systole. Because of this ridge, some of these squamates are capable of producing ventricular pressure differentials that are equivalent to those seen in mammalian and avian hearts.