How does transposition of the great arteries alter normal circulation?

In transposition of the great arteries, the normal circulatory pattern is fundamentally altered because the aorta and the pulmonary artery are connected to the incorrect ventricles.12

Specifically, the aorta arises from the right ventricle, which normally pumps deoxygenated blood to the lungs, while the pulmonary artery arises from the left ventricle, which normally pumps oxygenated blood to the body. Consequently, the right ventricle receives deoxygenated blood from the venae cavae and pumps it directly back into the systemic circulation, while the left ventricle receives oxygenated blood from the lungs and pumps it back into the pulmonary circulation.12

This creates a "short circuit" where oxygenated and deoxygenated blood do not mix properly to supply the body's tissues. Survival depends on a naturally occurring communication between the two sides of the heart—such as a septal defect—that allows oxygenated blood to enter the aorta; if no such connection exists, it must be created through medical or surgical intervention.12