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The atrioventricular (AV) valves stop blood flowing from the ventricles into the atria when the ventricles contract.
During ventricular systole, the atria relax and after a slight delay, the ventricles contract. The pressure in the ventricles increases and blood is forced upwards, closing the atrioventricular valves. Blood is forced into the aorta and pulmonary artery, opening the semilunar valves.
The pulmonary vein carries oxygenated blood from the lungs to the heart.
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