A septal defect is a hole in the wall (septum) that separates the heart into the left and right sides. Atrial septal defects are located between the heart's upper chambers (atria). Ventricular septal defects are located between the lower chambers (ventricles). In both types, some oxygenated blood, intended for the body, is shortcircuited. It is returned to the lungs rather than pumped to the rest of the body.