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Starting with (deoxygenated) blood entering the heart from the body/veins: Into the right atrium, then into the right ventricle, where it is pumped into the lungs to be enriched with oxygen. Then back into the left atrium and then into the left ventricle, pumped out through the aorta, (largest artery leaving the heart), to circulate back through the body/systems. Oxygen is delivered everywhere through the arteries, and now the oxygen-poor blood goes back up through the veins, into the vena cava, (largest vein that goes right into the right atrium), etc. etc.