The superior and inferior vena cava are the two main blood vessels that bring de-oxygentated blood back to the heart after the blood's done a circuit of the body to offload its oxygen to organs and muscles and tissues and cells, etc. The blood from these vessels enters the right atrium of the heart. Once the atrium is full of blood it contracts, pushing the blood through the open tricuspid valve into the right ventricle of the heart. The tricuspid valve closes shut once the blood is in the right ventricle, preventing the blood from returning to the right atrium. While the valve is closed, the ventricle contracts, pushing the blood out of the heart through the pulmonic valve, into the pulmonary artery, where it is carried directly to the lungs where it gets a new supply of oxygen attached to it. The freshly oxygenated blood courses along the pulmonary vein from the lungs back to the heart, entering the heart's left atrium. When the left atrium is full of blood, it empties by contraction, through the open mitral valve, into the left ventricle of the heart. The mitral valve then closes, so the blood cannot retrace its path. The left ventricle then contracts, pushing the blood out of the heart through the aortic valve into the big blood vessel called the aorta, from where it courses round the body delivering oxygen again.
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