In truncus arteriosus the aorta and pulmonary artery did not separate from each other as the heart was being formed. All the blood that leaves the right and left ventricle exit through one common vessel that then branches into the pulmonary arteries and the aorta. The valve (called the “truncal valve”) leading to the one common vessel is supposed to act as a one-way gate for blood, but can be too tight (stenotic) or leaky (regurgitant). There is also a hole in the ventricular septum (ventricular septal defect or VSD) allowing blood to mix between the right and left ventricle.