Heart: The muscle that pumps blood received from veins into arteries throughout the body. The heart is positioned in the chest behind the sternum (breastbone); in front of the trachea, esophagus, and aorta; and above the diaphragm. A normal heart is about the size of a closed fist and weighs about 298 grams or 10.5 ounces. It is cone-shaped, with the point of the cone pointing down to the left. Two-thirds of the heart lies in the left side of the chest, with the balance in the right side of the chest. The heart is composed of specialized cardiac muscle, and it is four-chambered, with a right atrium and ventricle, and an anatomically separate left atrium and ventricle. The blood flows from the systemic veins into the right atrium, thence to the right ventricle, from which it is pumped to the lungs and then returned into the left atrium, thence to the left ventricle, from which it is driven into the systemic arteries. The heart is thus functionally composed of two hearts: the right heart and the left heart. The right heart consists of the right atrium, which receives deoxygenated blood from the body, and the right ventricle, which pumps the deoxygenated blood to the lungs under low pressure; and the left heart, which consists of the left atrium, which receives oxygenated blood from the lung, and the left ventricle, which pumps the oxygenated blood out to the body under high pressure.