One can say that the reptile heart has three chambers, two atria and one, partially divided, ventricle. Or one may argue that reptiles have four-chambered hearts with two atria and two ventricles, but the wall between the ventricles is incomplete. In fact, both of these interpretations are correct. Though, as a result of the incomplete partition between the two ventricles, there is some mixing of oxygenated and deoxygenated blood, this is minimized by the timing of the contractions.
The crocodile is the lone exception among reptiles in terms of heart structure. In the crocodile heart the partition between the right and the left ventricle has been completed and resembles the four-chambered heart of birds and mammals. Nevertheless a peculiar difference from the four chambered hearts does exist. Specifically, the right ventricle pumps blood into not one but two separate vessels: the pulmonary artery and the, so-called, right systemic artery. Such an arrangement allows the crocodile to effectively switch between normal and low oxygen (apnea) conditions by closing the pulmonary circulation.
