Pulmonary arteries carry blood from the heart to the lungs. According to About.com, the primary pulmonary artery branches off the right ventricle of the heart before further branching into right and left pulmonary arteries on the way to the lungs. The right pulmonary artery is the longer of the two.
Typically arteries carry oxygen-rich blood to the various organs, and veins return the de-oxygenated blood back to the heart. In the case of the pulmonary arteries, however, they transport de-oxygenated blood to the lungs to pick up oxygen. The pulmonary veins bring the oxygenated blood back to the heart where it is then directed out to the rest of the body.