The lungs supply the circulation with oxygen. Oxygen-poor blood from the systemic circulation is collected in the right atrium of the heart. The right ventricle pumps the blood into the pulmonary arteries.
Oxygen is taken up into the blood of the alveolar capillaries. Oxygen-rich blood flows then back to the heart via the pulmonary veins into the left atrium. The left ventricle pumps the blood into the systemic circulation.