Normally there are two main blood vessels leaving the heart: the aorta, carrying blood to the body, and the pulmonary artery that branches immediately to carry blood to each lung.
Instead of having a separate pulmonary artery and aorta, each with its own three-leafed valves, a baby with truncus arteriosus has only one great blood vessel or trunk leaving the heart, which then branches into blood vessels that go to the lungs and the body.
This great vessel usually has one large valve which may have between two and five leaflets. Usually this great vessel sits over both the left and right ventricle. The upper portion of the wall between these two chambers is missing, resulting in what is known as a ventricular septal defect (VSD). In rare cases, the ventricular septal defect is absent.