< b>Heart: The muscle that pumps blood received from veins into arteries
throughout the body. It is positioned in the chest behind the sternum (breastbone;
in front of the trachea, esophagus, and aorta; and above the diaphragm muscle that
separates the chest and abdominal cavities. The normal heart is about the size of
a closed fist, and weighs about 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 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, 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 it to the lungs under low
pressure; and the left heart, consisting of the left atrium, which receives oxygenated
blood from the lung, and the left ventricle, which pumps it out to the body under
high pressure.
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