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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 fresh, oxygen-rich blood enters the pulmonary veins and then returns to the heart, re-entering through the left atrium. The oxygen-rich blood then passes through a one-way valve into the left ventricle where it will exit the heart through the main artery, called the aorta.he left ventricle's contraction forces the blood into the aorta and the blood begins its journey throughout the body. The one-way valves are important for preventing any backward flow of blood. The circulatory system is a network of one-way streets.