Atrial septal defect

The heart has 4 sections, or "chambers." The upper chambers are called "atria" and the lower chambers are called "ventricles." When the heart is working normally, blood comes in from the body through the right atrium and into the right ventricle. From there it goes to the lungs, where it picks up oxygen. Then the blood comes back through the left atrium, into the left ventricle, and back out to the body.
A person with an atrial septal defect (ASD) has a hole between the right atrium and the left atrium. This can change the way blood flows through the heart.