Pulmonary artery: One of the two vessels which are formed as terminal branches of the pulmonary trunk and convey un-aerated blood to the lungs. The two pulmonary arteries differ in length and anatomy. The right pulmonary artery is the longer of the two. It passes transversely across the midline in the upper chest and passes below the aortic arch to enter the hilum of the right lung as part of its root. The left pulmonary artery is the shorter of the two terminal branches of the pulmonary trunk. It pierces the pericardium (the sac around the heart) and enters the hilum of the left lung.