Deoxygenated blood is brought to the heart by the superior and inferior vena cava from the body. In the heart, the deoxygenated blood goes through the right atrium down into the right ventricle, which passes the blood through the pulmonary artery to the lungs. Here, gas exchange of CO2 (exhalation) and O2 (inhalation) occurs. The oxygenated blood goes back into the pulmonary veins to the left atrium which pumps into the left ventricle and up the aorta back to the body. Then the oxygenated blood is delivered to the tissues and the cycle starts over.