Illustration A shows the circulatory system of reptiles, which have a four-chambered heart. The right and left ventricle are separated by a septum, but there is no septum separating the right and left atrium, so there is some mixing of blood between these two chambers. Blood from systemic circulation enters the right atrium, then flows from the right ventricle and enters pulmonary circulation, where blood is oxygenated in the lungs. From the lungs blood travels back into the heart through the left atrium. Because the left and right atrium are not separated, some mixing of oxygenated and deoxygenated blood occurs. Blood is pumped into the left ventricle, then into the body. Illustration B shows the circulatory system of mammals, which have a four-chambered heart. Circulation is similar to that of reptiles, but the four chambers are completely separate from one another, which improves efficiency.
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