The left side of the heart pumps oxygen-rich blood to the body. If there is a hole in the wall of the heart, blood from the left and right sides can mix. The heart does not work as an efficient pump. Blood pumped to the body may lack adequate oxygen, or blood already rich in oxygen returns to the lungs. The heart will try to compensate by working harder.
Persistent overworking can cause it to enlarge.
Some septal defects are so small that they do not affect the heart in an important way. Others are so large or complex that they need to be closed with surgery. This diagram shows the opening between the two upper chambers, the atria , of the heart. Click here to continue.