Expansion
Septum primum and septum secundum overlap to leave the flap valve of the foramen ovale
Expansion
- Septum primum grows down from the roof of the common atrium, narrowing the ostium primum
- Before it closes, perforations appear in its upper part and coalesce as the ostium secundum
- Septum secundum grows down to its right, leaving an oval gap - the foramen ovale
- The remaining flap of septum primum acts as a one-way valve over that gap
In fetal life, higher right atrial pressure holds the flap open and blood crosses to the left atrium. At birth, the left atrial pressure rises and presses the flap shut.
- Secundum defect - the commonest atrial septal defect, from excessive resorption of septum primum or a deficient septum secundum
- Primum defect - a failure at the endocardial cushions, associated with atrioventricular valve abnormalities and with Down syndrome
- Patent foramen ovale - present in about a quarter of adults; a route for paradoxical embolism