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, it is through a patent ductus arteriosus.

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