The absorption of nutrients, elimination of metabolites and gas exchange during the fetal period are carried out by osmosis through the umbilical vessels and between the placenta and the mother. Blood with high oxygen content enters the body from the placenta through the umbilical vein and along the anterior abdominal wall of the fetus, and then divides into three branches: one directly into the liver; one converges with the portal vein into the liver, and blood from these two branches enters the inferior vena cava through the hepatic vein; the other is a venous catheter that enters the inferior vena cava directly. It can be seen that the inferior vena cava blood entering the right atrium is a mixture of blood from the umbilical vein with high oxygen content and venous blood from the lower part of the fetal body. Since the inferior vena cava opens in the right atrium directly opposite the foramen ovale, most of the highly oxygenated blood from the inferior vena cava flows into the left atrium through the foramen ovale. The blood from the superior vena cava into the right atrium, however, passes little or no through the foramen ovale and flows directly to the right ventricle into the pulmonary artery, which supplies the heart, head, and upper extremities first. After some blood from the left ventricle enters the descending aorta, it enters the placenta via a pair of umbilical arteries that divide from the external iliac artery for gas exchange with the maternal rest. The fetal blood circulation under these specific conditions ensures that blood with high oxygen saturation is supplied to the heart and brain in priority, while blood with low oxygen saturation is returned to the placenta. Such a specific blood circulation is accomplished through three main channels: the foramen ovale, the venous catheter and the arterial catheter. Under normal circumstances, all three channels are closed after birth.