Reptilian hearts, largely similar to amphibian hearts, are three-chambered and provide moderate separation of oxygenated and deoxygenated blood with their two atria, one which receives oxygenated blood and one which receives deoxygenated blood from the lungs. The reptilian heart's ventricle still allows oxygenated and deoxygenated blood to intermingle, meaning that it is not as robust or as efficient as a mammalian heart.
Separation of blood occurs because the arrival of blood is timed independently through the atria. If blood arrives through both atria simultaneously, then mixing occurs, but this may be to a greater or lesser degree depending on the animal's body temperature and state of activity.
Once blood reaches the conus arteriosus, it is drawn down the three channels of the truncus arteriosus, the entry to the circulatory system, and then proceeds to move through the rest of the reptilian body. The conus leads to three distinct systems, the pulmonary trunk system and the left and right systemic trunk systems.
Mammalian hearts are geared toward constant activity and high levels of movement and exertion while reptilian hearts perform better in short bursts and require that the animal rest frequently and work to control its body temperature. Lower levels of blood oxygen make reptiles sluggish and prone to sleep.