Aorta rising from the right ventricle and the pulmonary artery from the left ventricle, producing two noncommunicating circulatory systems (see Looking at transposition of the great vessels, page 234)

The transposed pulmonary artery carries oxygenated blood back to the lungs, rather than to the left side of the heart.

The transposed aorta returns the unoxygenated blood to the systemic circulation, rather than to the lungs.

Communication between the pulmonary and systemic circulation is necessary for survival; the presence of other congenital defects, such as atrial septal defect (ASD) or ventricular septal defect (VSD), is necessary to sustain life.