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Amphibians and reptiles, by contrast, have a three-chambered heart. The three-chambered heart consists of two atria and one ventricle. (The crocodile is sometimes said to have a four-chambered heart. The separation of the ventricles is not complete, however, because a hole remains in the septum (wall) that divides the two chambers.) Blood leaving the ventricle passes into one of two vessels. It either travels through the pulmonary arteries leading to the lungs or through a forked aorta leading to the rest of the body. Oxygenated blood returning to the heart from the lungs through the pulmonary vein passes into the left atrium, while deoxygenated blood returning from the body through the sinus venosus passes into the right atrium. Both atria empty into the single ventricle, mixing the oxygen-rich blood returning from the lungs with the oxygen-depleted blood from the body tissues. While this system assures that some blood always passes to the lungs and then back to the heart, the mixing of blood in the single ventricle means the organs are not getting blood saturated with oxygen. This is not as efficient as a four-chambered system, which keeps the two circuits separate, but it is sufficient for these cold-blooded organisms.

Whereas

Amphibians and reptiles, by contrast, have a three-chambered heart. The three-chambered heart consists of two atria and one ventricle. (The crocodile is sometimes said to have a four-chambered heart. The separation of the ventricles is not complete, however, because a hole remains in the septum (wall) that divides the two chambers.) Blood leaving the ventricle passes into one of two vessels. It either travels through the pulmonary arteries leading to the lungs or through a forked aorta leading to the rest of the body. Oxygenated blood returning to the heart from the lungs through the pulmonary vein passes into the left atrium, while deoxygenated blood returning from the body through the sinus venosus passes into the right atrium. Both atria empty into the single ventricle, mixing the oxygen-rich blood returning from the lungs with the oxygen-depleted blood from the body tissues. While this system assures that some blood always passes to the lungs and then back to the heart, the mixing of blood in the single ventricle means the organs are not getting blood saturated with oxygen. This is not as efficient as a four-chambered system, which keeps the two circuits separate, but it is sufficient for these cold-blooded organisms.

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