Fine.
ASD is when there's a hole between the right and left atrium. This is physiologic in the newborn who needs it to shunt oxygenated blood from the inferior vena cava into the left atrium and to the aorta so that the brain and upper extremities can get the oxygen they need. This hole is made by the septum primum because the cells can't quite reach the rest of the endocardial cushion and leaves a hole called the ostium primum. Then they realize their mistake and some of the cells move down to cover the hole but leave open a hole called the ostium secundum. Then another group of cells come down and form the septum segundum, closer to the right atrium. They have the same problem as the first septum but don't move down so that there's a hole between the ostium secundum and the foramen ovale. Normally, when a newborn is born, the force of the first breath clears out the fluid from the alveoli into the interstitium and the increased oxygen tension leads to a closing of the pda. Opening the alveoli means more oxygen gets into the pulmonary vasculature which increases the oxygen tension and decreases resistance. Remember that lower oxygen tension decreases vascular resistance in most other systems because if they have low oxygen, then the cells need more blood flow to get enough oxygen to survive. But in the lungs, if you close off the blood supply to where there is low oxygen tension, and sends the blood to areas of high oxygen tension so that they can be more efficient at picking up oxygen.
So when the pulmonary vascular resistance goes down, more blood goes through the pulmonary system and the pressure differential between the right and the left atrium increases which normally "closes" the foramen ovale by pressing it up against the septum primum. The hole is still there which is visible as the fossa ovale. Sometimes the membranes don't fuse and that's called a patent foramen ovale which can lead to a paradoxical embolus. Ostium primum defect is when the septum primum doesn't close because there's no fusion of the superior and inferior endocardial cushions in the anterior, inferior aspect of the atrial septum. So where there is supposed to be a bunch of tissue that fills in the space where the foramen ovale should be in the septum secundum, there's a hole so this is a ostium primum ASD. This is associated with down syndrome. Ostium secundum is an asd in the upper portion of the atrial septum because the tissue underestimated how much there needs tobe to cover the hole of the foramen ovale. This problem can be due to an enlarged foramen ovale (so that it is not completely covered by the septum primum), inadequate growth of the septum secundum (so there's a part of the hole that's not covered) or excessive absorption of the septum primum.
