In the left atrium, this blood mixes with a relatively small quantity of pulmonary venous blood, enters the left ventricle, and then proceeds to the coronary circulation and vessels supplying the head, neck, and upper extremities. Most of the blood crossing the foramen ovale corresponds to the stream of well-oxygenated blood in the inferior vena cava coming from the ductus venosus.

The inferior vena cava blood enters the right atrium, and approximately 40% is diverted to the left atrium through the foramen ovale. However, this mixture is not complete, and a streaming of blood within the proximal inferior vena cava has been described. The saturation of blood in the inferior vena cava is lower than that in the ductus venosus, because it has mixed with poorly oxygenated blood returning from the lower body. An average of 50% takes the latter path, but the proportion is variable. Some of this blood is shunted directly to the inferior vena cava through the ductus venosus, while some traverses the hepatic parenchyma. The umbilical vein enters the liver, where it joins with the portal venous system. Well-oxygenated blood returns to the fetus from the placenta by way of the umbilical vein.

Heart blood flow print