Truncus arteriosus is a defect in the large blood vessels that leave the heart.

Normally, two large blood vessels, called the aorta and pulmonary artery carry blood away from the heart. The aorta carries oxygen-rich blood from the heart to the rest of the body. The pulmonary artery carries oxygen-poor blood from the heart to the lungs. As the heart develops. a section of these two blood vessels sometimes combine together. It creates one large vessel called the truncus arteriosus. The oxygen-poor and oxygen-rich blood leaving the heart mix in this combined blood vessel. The mixed blood decreases the amount of oxygen-rich blood that reaches the body.

The defect also includes a large hole in the wall between the lower chambers of the heart.

Heart Chambers and Valves
heart anatomy
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Blood Flow Through the Heart
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Truncus arteriosus is a problem with the development of the heart while the baby is in the womb. It is not known exactly why some hearts develop this way.