Mammals have closed, double-loop circulatory systems with four-chambered hearts.
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In mammals, blood travels through a system of closed vessels to and from the heart. There are two loops: one that brings oxygen-poor blood to the lungs to fill up on oxygen and another that pumps oxygen-rich blood to the rest of the body. This requires that there are essentially two "hearts." The right atrium receives deoxygenated blood from the body, sends it to the ventricles, where it is pumped to the lungs to pick up oxygen. This oxygen-rich blood returns to the heart by way of the left atrium, is transferred to the ventricle, and is then pumped to the rest of the body to deposit its oxygen supply at the various organs and tissues.