A disruption in the blood supply to the brain starves the brain of oxygen-rich blood and causes the nerve cells in that area to become damaged and die within minutes.
If the defect is large, oxygen-rich blood from the left atrium flows back into the right atrium and gets pumped back to the lungs again, causing more work for the heart and lungs.
If the heart has an opening between the atria, oxygen-rich blood from the left atrium enters the right atrium.
The defect allows oxygen-rich blood from the lungs to flow from the left side of the heart to the right side of the heart and back again to the lungs.
Total anomalous pulmonary venous return is a condition in which the pulmonary veins that bring oxygen-rich blood from the lungs back to the heart are not connected to the left atrium.