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A vital, muscular organ in higher animals that pumps blood around the body. In vertebrates, (animals with backbones) the heart has chambers. Fish have a two-chambered heart, reptiles a three-chambered heart. Humans, other mammals, and birds have a four-chambered heart. In humans, the heart has two chambers, called auricles or atria, that receive blood and two chambers, called ventricles, that pump blood.