The lungs help exchange the deoxygenated blood which are full of wast materials into oxygenated and blood full of nutrients. These oxygenated blood are then transported through the pulmonary vein, the only vein in the body which transports oxygenated blood. These blood are then transported back into the heart, which is the left atrium specifically.
Those blood is then pushed down to the left ventricle, and then pumped to the whole body (except lungs), in order to keep the body functioning by providing oxygen and nutrients for cellular respiration.
Meanwhile, the deoxygenated blood are received by the right atrium, and is then transported back to the lungs for exchange again, through the right ventricle and pulmonary artery.
