There are two circulatory pathways, the major circulation and the minor circulation. The major circulation starts from the left ventricle, and when the left ventricle contracts, the blood from the left ventricle goes to the aorta, and through the aorta to the various branches of the arteries. In this way, it reaches all tissues of the body and exchanges substances with the cells of the body, leaving the aerobic substances behind and entering the metabolic products and carbon dioxide into the blood to form venous blood. The small circulation starts from the right ventricle, which contracts and pumps the metabolic products and carbon dioxide into the pulmonary artery, where the branches of the pulmonary artery exchange with the oxygenated substances in the lungs. This expels carbon dioxide and brings in oxygen, which increases the oxygen in the blood and flows back into the left atrium through the pulmonary veins.