Explain how blood is pumped around the body

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Oxygenated blood flows into the left atrium from the lungs via the pulmonary vein, when the left atrium contracts the blood flows down into the left ventricle via the mitral valve (which divides the atrium from the ventricles). When the right ventricle contracts the blood is squeezed up into the aorta, which is the main artery carrying oxygenated blood to all of the rest of the body, lots of smaller arteries branch off of the aorta to deliver oxygen and nutrients to all of the tissues of the body. These tissues take the oxygen from the blood in exchange for carbon dioxide, meaning that the blood flowing back to the heart is deoxygenated; this deoxygenated blood is carried back to the heart in the veins. All of these veins empty into the vena cava, which enters the right atrium of the heart. When the right atrium contracts it squeezes the blood into the right ventricle, via the tricuspid valve. The right ventricle then contracts, squeezing the deoxygenated blood into the pulmonary artery to be carried to the lungs. When the deoxygenated blood arrives in the lungs (which are full of atmospheric air and therefore oxygen) the opposite thing happens to what happened in the tissues; the carbon dioxide is exchanged for oxygen, so that the blood becomes oxygenated again and travels back to the heart via the pulmonary vein to be delivered into the left atrium once again, for a new cardiac cycle. 

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