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Pulmonary circulation is the movement of blood from the heart, to the lungs, and back to the heart again. De-oxygenated blood leaves the heart, goes to the lungs, and then re-enters the heart; oxygen poor blood leaves through the right ventricle through the pulmonary artery, the only artery in the body that carries oxygen-poor blood, to the capillaries where carbon dioxide diffuses out of the blood cell into the alveoli, and oxygen diffuses out of the alveoli into the blood. Blood leaves the capillaries to the pulmonary vein, the only vein in the body that carries oxygen-rich blood in the body, to the heart, where it re-enters at the left atrium.
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