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The left pulmonary artery originates at the division of the pulmonary trunk anterior to the left main bronchus. For about 1cm it is ensheathed within the pericardium as it passes superolaterally. It curves around the left main bronchus within the lung root to lie on its superior aspect at the hilum. At the hilum, where it is the most superior structure, it gives off its first branch which passes to the left superior lobe.
Passing inferolaterally and posterior to the superior lobe bronchus, the left pulmonary artery then gives off a 'lobar' branch to the lingula. It then runs vertically down, posterior to other branches of the bronchial tree, to form the lobar and then segmental bronchi of the left inferior lobe. Successive divisions of the branches eventually produce capillaries in the walls of alveoli.
The ligamentum arteriosum is attached to the superior surface of the left pulmonary artery just distal to its point of origin. The pericardial fold of the vestigial left vena cava passes from the inferior margin of the left pulmonary artery; it passes to the upper left pulmonary vein.
The left pulmonary artery is shorter and of smaller diameter than the artery on the right side.