Oxygenated blood reaches the fetus via the umbilical vein (PaO₂ 4.7kPa; SaO₂ 80-90%). Up to 60% of this blood flow bypasses the liver in the ductus venosus (DV) which then becomes the inferior vena cava (IVC).

At the junction of the IVC and the right atrium (RA) a tissue flap directs the more highly oxygenated blood, streaming along the dorsal aspect of the IVC, across the foramen ovale (FO) and into the left atrium (LA). In the LA, the oxygen saturation of fetal blood is 65%.

This better oxygenated blood enters the left ventricle (LV) and is ejected into the ascending aorta. The majority of the LV blood is delivered to the brain and coronary circulation thus ensuring that blood with the highest possible oxygen concentration is delivered to these vital structures.

Oxygen delivery is dependent on haemoglobin concentration, cardiac output and oxygen saturation.

The cardiac output of the fetus is regarded as the total output of both ventricles - the combined ventricular output (CVO). About 45% of the CVO is directed to the placental circulation with only 8% of CVO entering the pulmonary circulation. The presence of fetal haemoglobin and a high CVO will help to maintain oxygen delivery despite low PaO₂.

With the high pulmonary vascular resistance and the presence of the ductus arteriosus most of the right ventricular output passes into the arch of the aorta just distal to the origin of the arteries to the head and upper limbs.