An atrial septal defect or ASD is essentially a hole in the wall that separates the heart's two top chambers. The hole in question forms in the septum or atria and, as a result, oxygen-rich blood from the heart can leak into oxygen-poor chambers of the lungs.
Every child is born with an opening between the atria that helps to direct blood from the lungs while the fetus is in development. Once the child is born, the opening is no longer necessary and will close within weeks to months of birth. Sometimes, this opening doesn't close completely. As a result, the child will have an atrial septal defect.
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