Before birth, the fetal circulatory system includes three open structures through which blood moves. These pathways normally close soon after birth and include:
Ductus arteriosus
Ductus venosus
Foramen ovale
The foramen ovale allows the shunting of oxygen rich blood from the placenta via the ductus venosus into the left atrium.
IN UTERO
The umbilical vein brings oxygenated blood through the ductus venosus to the inferior vena cava and right atrium, where it is directed across the patent foramen ovale into the left atrium. This allows the left ventricle to pump the most oxygenated blood to the coronary arteries (heart) and carotid arteries (brain).
Deoxygenated blood returning via the superior vena cava is directed across the tricuspid valve and into the right ventricle. The right ventricle pumps this blood into the pulmonary arteries, across the patent ductus arteriosus into the descending aorta.
AT BIRTH
The lungs inflate when the baby takes his/her first breath. This decreases pulmonary vascular resistance and increases the flow of blood from the right ventricle. The resultant increase in pulmonary blood flow leads to an elevation in left atrial pressure, which causes the septum primum flap to seal the foramen ovale, closing this hole and separating the two atria. This also increases blood flow to the lungs, as blood entering the right atrium can no longer bypass the right ventricle and is now pumped through the pulmonary artery into the lungs.
In premature infants, sometimes the septum primum flap fails to seal the foramen ovale after birth and the shunt remains open. This is termed a patent (or open) foramen ovale (PFO). As the heart grows in size, the PFO also increases in size.
The PFO opening offers the potential for the shunting of blood between the left and right atria. In most cases, however, there are no associated symptoms and no treatment is prescribed. Eventually, the opening will close on its own.
