Dictionary of Human Evolution and Biology

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Hollow muscular organ that pumps blood through the circulatory system; positioned in the thoracic cavity, usually slightly to the left of mid-line (Cf. situs inversus viscerum). In mammals, birds and Crocodilia, the heart has four chambers and pumps blood to two circuits, the pulmonary circuit (to and from the lungs) and the systematic circuit (to the rest of the body). The four-chambered heart is more efficient at delivering highly oxygenated blood to the tissues than is the three-chambered heart.

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From "Dictionary of Nursing and Individual Health Care"