The blood pumped by the right ventricle enters the pulmonary artery, whereas the left ventricle pumps blood into the aorta. The deoxygenated blood pumped into the pulmonary artery is passed onto the lungs from where the oxygenated blood is carried by the pulmonary veins into the left atrium. Thus the blood leaving lungs is richer in oxygen as compared to blood entering the lungs. This pathway constitutes the pulmonary circulation.

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