In normal circulation, the pulmonary artery arises from the right ventricle and the aorta arises from the left ventricle, which are separate from each other. Coronary arteries, which supply the heart muscle, arise from the aorta, just above the valve at the entrance of the aorta.
In truncus arteriosus, a single arterial trunk arises from the ventricles. A large ventricular septal defect (hole between the two ventricles) is usually present along with a truncus defect, essentially making the right and left ventricles into a single chamber, with the blue (unoxygenated) and red (oxygenated) blood mixing completely. Some of this mixed blood goes to the lungs, some to the coronary arteries, and the rest to the body. Whether more blood goes to the body versus the lungs is determined by the pressures in the two places. Generally the pressure is much lower in the lungs than in the body, so too much blood gets pumped to the lungs, flooding them with fluid.
If left untreated, this increased blood flow causes two problems. First, the lungs are filled with fluid, making it difficult to breathe. The second problem is more long term. When the lungs get too much blood, the vessels there become narrowed and are permanently damaged, to the point where they are so narrow the heart can no longer force blood into them. This is called pulmonary hypertension and it can be life-threatening.
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