How is the circulatory system of an amphibian characterised?

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Amphibians have a three-chambered heart comprised of one ventricle and two atria. Thus, amphibians have a left and right atrium but a single large ventricle. This is different from mammals that have a four-chambered heart with two atria and two ventricles. The ventricle in amphibians is partially separated due to some space left along the separating wall. Blood from the ventricle has two pathways: it can either flow through the pulmonary artery heading to the lungs or through the aorta to the rest of the body. Blood that has been oxygenated in the lungs flows back to the heart through the pulmonary vein and into the left atrium. Deoxygenated blood from the rest of the body flows back to the heart through the sinus venosus and into the right atrium. The left and right atria empty both the oxygenated and deoxygenated blood into the ventricle. The amphibian circulatory system is not as effective as that of mammals in keeping oxygenated and deoxygenated blood flowing separately, but the system is sufficient for the organisms.

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