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Blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium. Left Side. Oxygen-rich blood returns from the lungs to your heart's left atrium through the pulmonary veins.he mitral valve does this between the left atrium and ventricle. The lub is the sound of the tricuspid and mitral valves closing. Each of your heart's ventricles has an outlet valve. The right ventricle uses the pulmonary valve to help move blood into the pulmonary arteries.
From there the blood enters the pulmonary artery and end up in the lungs. After it gets oxygen rich, the blood is sent back to the heart through the pulmonary veins. The blood is dumped into the left atrium and there is another special valve between the left atrium and the left ventricle. The valve closes down so that the blood doesn't go back to the atrium when it squeezes.