The pulmonary artery [a. pulmonalis] passes from the right ventricle to the lungs. It differs from all other arteries in the body in that it contains venous blood. It arises as a short, thick trunk from the conus arteriosus of the right ventricle, and, after a course of about 5 cm. (2 in.) within the pericardium, divides into a right and a left branch. These branches pass to the, right and the left lung respectively.

Trunk of the pulmonary artery

The trunk of the pulmonary artery at its origin is on a plane anterior to the ascending aorta, and slightly overlaps that vessel. Thence it passes upward, backward, and to the left, forming a slight curve around the front and left side of the ascending portion of the aorta; and, having reached the concavity of the aortic arch, on a plane posterior to the ascending aorta, it divides into its right and left branches, which diverge from each other at an angle of about 130°. The division of the pulmonary artery occurs immediately to the left of the second left chondrosternal articulation.

In the fetus, the pulmonary artery continues its course upward, backward, and to the left under the name of the ductus arteriosus (Botalli), and opens into the descending aorta just below the origin of the left subclavian artery. After birth, that portion of the pulmonary artery which extends to the aorta becomes obliterated, and remains merely as a fibrous cord, the ligamentum arteriosum.

Relations

In front, the trunk of the pulmonary artery is covered by the remains of the thymus gland, and the pericardium. The artery lies, at its commencement, behind the upper margin of the third left chondro-sternal articulation. The right margin of the artery is behind the second piece of sternum but the greater part of the vessel is behind the medial end of the second intercostal space.

Behind, it lies successively upon the ascending aorta and the left atrium.

To the right are the ascending aorta, the right atrium, the right coronary artery, and the cardiac nerves.

To the left are the pericardium, the left pleura and lung, the left auricle, the left coronary artery, and the cardiac nerves.

The right pulmonary artery

The right pulmonary artery [ramus dexter] longer than the left, passes almost horizontally under the arch of the aorta to the root of the right lung, where it divides, either directly or after repeated division, into three branches, one for each lobe. These branches follow the course of the bronchi, dividing and sub- dividing for the supply of the lobules of the lung. The terminal branches do not anastomose with each other.

Relations of the right pulmonary artery

In its course to the lung it has in front of it the ascending aorta, the superior vena cava, the phrenic nerve, the anterior pulmonary plexus, and the reflexion of the pleura. Behind are the right bronchus and the termination of the azygos vein. Above is the arch of the aorta, and below are the left atrium and the upper right pulmonary vein.

At the root of the lung it has the right bronchus above and behind it; the pulmonary veins below and in front. Crossing in front of it and the other structures forming the root of the lung are the phrenic nerve and the anterior pulmonary plexus; behind are the azygos vein, the vagus nerve, and the posterior pulmonary plexus.

The left pulmonary artery

The left pulmonary artery, shorter and slightly smaller than the right, passes in front of the descending aorta to the root of the left lung, where it divides into two branches for the supply of the upper and lower lobes respectively. These divide and subdivide as on the right side.

Relations of the left pulmonary artery

At the root of the lung it has the left bronchus behind and also below it, in consequence of the more vertical direction taken by the left bronchus than by the right. Below and in front are the pulmonary veins, while passing from the artery and the upper left pulmonary vein is the ligament of the left superior cava. Crossing in front of it and the other structures forming the root of.the lung are the phrenic nerve, the anterior pulmonary plexus, and the reflex- ion of the left pleura; crossing behind, are the descending aorta, the left vagus nerve, and the posterior pulmonary plexus.