The deoxygenated blood enters the heart through the right atrium then goes to the right ventricle. It passes through the pulmonary artery and goes to the lungs where the exchange of gases takes place specifically at the alveoli. It removes the carbon dioxide in the blood and replaces it with oxygen. Then the oxygenated blood passes through the pulmonary vein and enters the heart through the left atrium, then to the left ventricle and to the aorta to all parts of the body.