The remaining opening is called the foramen ovale. The upper portion of the septum primum is gradually resorbed and the remaining lower portion becomes the valve of the foramen ovale or foraminal flap. The foramen ovale is one of three communication pathways in the fetus that allows for adequate exchanging of gases between the fetus and placenta. The ductus venosus and ductus arteriosus are the other two. In fetal life, shunting of blood from the right to the left atrium, through the foramen ovale, is physiologic.
Circulation begins with oxygenated blood entering the fetus through the umbilical vein.
The majority of this blood bypasses the liver through ductus venosus and enters the inferior vena cava. The IVC contains a mixture of oxygenated blood, and deoxygenated blood from the lower extremities. Most of this blood is shunted into the left atrium through the foramen ovale. A small amount of deoxygenated pulmonary venous blood converges with the left atrium, and mixes with its oxygen plentiful blood. This blood descends into the left ventricle and exits the heart through the ascending aorta.