A PFO is a normal structure in the fetal heart before birth.  It is actually a flap in the heart wall that separates the top two chambers of the heart (the right atrium - RA and the left atrium - LA). In most people, the flap shuts and is sealed after birth.  But in 20 - 25% of the population, the flap does not seal completely, and can continue to open throughout a person's lifetime (top video), allowing communication between the right and left sides of the heart.

 

The right side of the heart receives all of the "dirty" or "used" blood back from the body, and pumps it to the lungs to have it filtered and cleaned. The cleaned blood then flows back to the left side of the heart, which pumps it all over the body, bringing nutrients and oxygen to the muscles, the organs and the brain.  

 

When a PFO is present, some of the "dirty" blood can cross to the "clean" side of the heart (bottom video).  In this "bubble study", salt water injected into the vein, from an intravenous line, appears as bubbles in the right atrium and crosses to the left atrium when the PFO opens.