Pulmonary arteries. Coloured magnetic resonance imaging (MRI) scan of the pulmonary arteries, the only arteries to carry deoxygenated blood. The main pulmonary artery, or pulmonary trunk (centre), starts at the right ventricle of the heart. It then branches into the left and right pulmonary arteries, which carry the deoxygenated blood to the corresponding lung. In the lungs gaseous exchange takes place, where the waste carbon dioxide leaves the blood and oxygen enters it. The oxygenated blood then returns to the heart to be pumped around the body.