Arteries of the small (pulmonary) circulation. Pulmonary trunk. The pulmonary trunk, truncus pulmonalis, carries venous blood from the right ventricle to the lungs. It is a continuation of the truncus arteriosus and is directed obliquely to the left, crossing the aorta lying behind it. The location of the pulmonary trunk in front of the aorta is explained by the fact that the truncus pulmonalis develops from the ventral part of the truncus arteriosus, and the aorta from the dorsal. Having passed 5 – 6 cm, the pulmonary trunk is divided under the aortic arch at the level of the IV – V thoracic vertebra into two terminal branches – a. pulmonalis dextra and a. pulmonalis sinistra, each going to the corresponding lung. The right and left pulmonary arteries develop from 6 arterial arcs laid in the embryonic life. The right one, longer, passes to the right lung behind aorta ascendens and the superior vena cava, the left one in front of aorta descendens. Passing to the lungs, a. pulmonalis dextra and a. pulmonnalis sinistra are again divided into branches to the corresponding lobes of the lungs and to the pulmonary segments and, accompanying the bronchi, branch into the smallest arteries, arterioles, precapillaries and capillaries. To the site of division, truncus pulmonalis is covered with a leaf of pericardium. From the site of division to the concave side of the aorta, the connective tissue cord stretches – lig. arteriosum, which is obliterated ductus arteriosus.