the left pulmonary artery is shorter and smaller than the right. It is also the most superior  of the pulmonary vessels . it travels horizontally arching over the left mainstem bronchus and enters the hilum of the left lung just superior to the left mainstem bronchus. Within the lungs each pulmonary artery desends posterolateral to the main bronchus and divides into lobar and segmental arteries continuing to branch out and to follow along with the smallest divisions of the bronchial tree.