Truncus arteriosus (common arterial trunk) means that there is one common vessel or trunk from the heart instead of a separate pulmonary artery and aorta. There is only one valve between the heart and this trunk. Normally, the pulmonary artery carries the pulmonary circulation — the blood that goes to the lungs, and the aorta carries the systemic circulation — the blood that goes throughout the rest of the body.
In truncus arteriosus, the trunk carries both the pulmonary and systemic circulation, as well as the coronary circulation, which normally arises off the aorta. The trunk overrides both ventricles, and there is always a ventricular septal defect (VSD), a hole in the wall between the lower two chambers of the heart. In truncus arteriosus, the oxygenated blood mixes with unoxygenated blood in the heart through the VSD. All the blood goes through the trunk and can either proceed through the trunk or go into the pulmonary artery and go to the lungs without ever carrying oxygen to the body.
Heart with Truncus Arteriosus