Spiral aorticopulmonary septum divides the truncus arteriosus into aorta and pulmonary artery. The spiral nature of the septum causes the root of aorta to be on the left side while the ascending aorta is on the right side of the pulmonary trunk. Since this septum also contributes to the development of the interventricular septum, persistent truncus arteriousus is associated with a ventricular septal defect.