In truncus arteriosus, one large vessel comes out of the heart, instead of two separate ones. There's also usually a hole in the wall between the lower heart chambers, called the ventricles. The hole is called a ventricular septal defect. In truncus arteriosus, oxygen-rich blood, shown in red, and oxygen-poor blood, shown in blue, mix together. The mixed blood is shown in purple. It doesn't contain enough oxygen for the body's needs.
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