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While most reptiles have 3-chambered hearts, the heart of alligators, and all crocodilians, has 4 chambers, a trait shared with mammals and birds. The advantage of a 4-chambered heart is that oxygenated blood and deoxygenated blood are separated, which results in more efficient respiration needed for the high metabolism of endothermic (warm-blooded) animals, and enables different pulmonary (lung) and systemic blood pressures, but is seemly over-complex for ectothermic (cold-blooded) crocodilians. The single ventricle of the 3-chambered reptile heart allows some mixing of oxygenated blood with deoxygenated blood, which may help regulate their metabolic state. Crocodilians have evolved a shunt between the left and right aorta (immediately above the ventricles) to facilitate the mixing of oxygenated and deoxygenated blood. Crocodilians also have a valve in the pulmonary artery that, when closed, forces deoxygenated blood to recirculate through the left aorta, which increases mixing. This increased mixing helps crocodilians transition to a lower metabolic state, and enables them to dive for extended periods. Some scientists have hypothesized that the complex heart structure of crocodilians might indicate that they evolved from endothermic (warm-blooded) ancestors.