The pulmonary trunk is one of the major arteries leaving the heart; it is about 5 centimeters (2 inches) long and 3 centimeters (1.2 inches) in diameter. The pulmonary trunk lies within the fibrous pericardium, enclosed, with the ascending aorta, in a sheath of the serous pericardium. It begins at the upper end of the infundibulum, behind the sternal extremity of the third left costal cartilage; and it runs backward and upward, winding round the left side of the ascending aorta, into the concavity of the aortic arch, where it bifurcates to form the right and left pulmonary arteries. The bifurcation takes place opposite the sternal end of the second left costal cartilage, so that the main part of the vessel is behind the sternal end of the second left intercostal space.

As it passes from the front of the aorta to its left side, the pulmonary trunk comes to lie in front of the upper part of the left atrium, from which it is separated by the transverse sinus of the pericardium. Anteriorly, the pericardium separates it from the anterior part of the left pleura and lung. At its origin, it has a coronary artery on each side of it.