The blood from the body enters the right upper collecting chamber (right atrium or RA) and from there, passes through a valve to the right-sided pumping chamber (right ventricle or RV). 

The right ventricle (RV) then pumps the blood to the lungs where the blood picks up oxygen and turns a bright red color.  The lungs also filter the blood of small clots and other debris.

The red, oxygen-rich blood then returns from the lungs to the left upper collecting chamber (left atrium or LA) and from there passes through a valve to the left-sided pumping chamber (left ventricle or LV)

The left ventricle (LV) then pumps the oxygen-rich, red blood to the aorta which is the large blood vessel that carries the blood to the body.

The atrial septum is the name of the wall that divides the right atrium (RA) and the left atrium (LA). Septum is the latin word for wall.

The atrial septum is made of two parts.  In reality, the wall is more like a door.  In everyone, this door is open before they are born.  The door closes shortly after birth.  In most cases, over the first few years of life, the door seals shut becoming a true wall between the right and left upper chambers. 

In 10-30% of the adult population, the "door" of the atrial septum closes, but does not seal shut and can still be opened briefly at times.  Things that tend to open the door include deep beathing, coughing, or straining.

If the door of the PFO opens, blue blood may pass to the left atrium, and go to the body without first passing through the lungs.  Because the lungs normally act as a filter for small clots and other debris, those with a PFO are at-risk of having clots and debris go to the body without first passing through the protective filter of the lungs.

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