Foramen ovale

From Ganfyd

In foetal life the foramen ovale is patent and allows blood to flow between the atria, bypasing the collapsed lungs. After birth the changing haemodymnamics push it closed and in many people it sticks that way. In more people than used to be thought, however, changes in pressure can reopen it and flow occur in what would in foetal life have been retrograde fashion.

(Persistently) Patent foramen ovale (PFO) can allow useful therapeutic efforts within the heart, and rarely "paradoxical" embolism causing stroke. The main article on this subject is on atrial septal defects, large ones of which are a far more important cause of strokes than PFOs.