The venous blood flowing back to the right atrium from the systemic circulation through the superior and inferior vena cava flows through the tricuspid valve and continues to reach the right ventricle. The venous blood emitted from the right ventricle enters the pulmonary artery, passes through the branches of the pulmonary artery at all levels in the lung, and finally carries out gas exchange in the capillary network to turn the venous blood into oxygen-rich arterial blood, passes through the branches of pulmonary veins at all levels in the lung, and then reaches the left atrium through the pulmonary vein.
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