||The Foramen Ovale in Fetal Circulation
Before birth, the fetal circulatory system includes three open structures through which blood moves that normally close soon after birth. These structures are the ductus arteriosus, the ductus venosus, and the foramen ovale. The foramen ovale allows the shunting of blood from the right atrium into the left atrium.
In the fetus, oxygenated blood (red in the illustrations) enters the circulatory system through placental transfer. The umbilical vein brings oxygenated blood through the ductus venosus to the inferior vena cava and right atrium, where it is directed by the Eustachion valve across the patent foramen ovale and into the left atrium. This allows the left ventricle to pump the most oxygenated blood to the coronary and carotid arteries.
Deoxygenated blood (blue in the illustrations) returning via the superior vena cava is preferentially directed across the tricuspid valve and into the right ventricle. The right ventricle pumps this blood into the pulmonary arteries, and across the patent ductus arteriosus into the descending aorta.
The foramen ovale is an opening in the atrial septum, formed by a flap of septum primum that is pushed open by flow of blood from the right to left atria. Oxygenated blood from the ductus venosus is directed by the Eustachian valve toward the foramen ovale, and into the left heart. This shunts the blood with the highest oxygen content to the left heart for perfusion of the coronary arteries and brain. From the left atrium, the oxygenated blood is pumped into the left ventricle and into the aorta, which carries it to the body. From there it returns to the placenta via the umbilical arteries.