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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.
The oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. This blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. From the left ventricle the blood passes through the aortic valve to the aorta.