Heart, transposition great vessels, diagram CV104.jpg In the diagram above, transposition of the great vessels is shown. This occurs when the trunco-conal septum does not spiral down. Instead, it descends straight down. As a result, the outflow of right ventricle is into the aorta and the outflow from the left ventricle is into the pulmonic trunk.In order for this system to work, there must be a connection between the system and pulmonic circulations. Sometimes this is through a ventricular septal defect or an atrial septal defect. In the diagram at the left, this is through a patent ductus arteriosus.