Throughout intrauterine life, maternal blood supplies the fetus with oxygen and nutrients, and carries away its wastes. The anatomical structures that allow for such an exchange are the umbilical blood vessels. The umbilical vein transports oxygenated blood and nutrients from the placenta to the fetus. The umbilical vein enters the body through the umbilical ring and travels along the anterior abdominal wall to the liver inside the ventral mesentery, which in this region later forms the falciform ligament of the liver. About 50% of the blood it carries passes into the portal venous system of the liver. The remaining 50% enters a vessel called the ductus venosus which bypasses the liver. The ductus venosus travels a short distance and joins the inferior vena cava. There, the oxygenated blood from the placenta is mixed with the deoxygenated blood from the fetal lower extremities, pelvis and kidneys. Inside the ductus venosus, there is a sphincteral mechanism able to control the blood flow to the fetal heart in stress situations as well as during uterine contractions.
This mixture of oxygenated and deoxygenated blood continues through the inferior vena cava to the right atrium. A large amount of blood is shunted from the right atrium through the foramen ovale (an opening between right and left atrium that directs the flow right to left) and into the left atrium. The septum primum, located on the left side of the atrial septum and overlying the foramen ovale, helps prevent blood from moving in the reverse direction. A smaller amount of blood, however, is stopped from entering the foramen by the septum secundum or the crista dividens, and remains in the right atrium to mix with the deoxygenated blood returning from the head and upper limbs via the superior vena cava. The right atrium also receives blood from the superior vena cava. Blood from the right atrium, passes into the right ventricle and out through the pulmonary trunk. However only five to ten percent of the overall cardiac output goes to the lungs, because most of the blood in the pulmonary trunk bypasses the lungs by entering a fetal vessel called the ductus arteriosis, which connects the pulmonary trunk to the descending portion of the aortic arch. In fetal life, in fact, only a small volume of blood, necessary for oxygen supply, enters the pulmonary circuit, as the fetal lungs are collapsed, and their blood vessels have a high resistance to flow.
The more highly oxygenated blood that enters the left atrium through the foramen ovale is mixed with a small amount of deoxygenated blood returning from the pulmonary veins. This mixture moves into the left ventricle and is pumped into the aorta. Some of it reaches the myocardium through the coronary arteries and some reaches the brain through the carotid arteries. The blood carried by the descending aorta is partially oxygenated and partially deoxygenated. Some of it is carried into the branches of the aorta that lead to various parts of the lower regions of the body. The rest passes into the umbilical arteries (usually two), which branch from the fetal internal iliac arteries and lead to the placenta.
Inflation of the lungs at birth reduces the resistance to blood flow through the lungs resulting in increases blood flow from the pulmonary arteries. Consequently, an increased amount of blood flows from the right atrium to the right ventricle and into the pulmonary arteries and less blood flows through the foramen ovale to the left atrium. In addition, an increased volume of blood returns from the lungs through the pulmonary veins to the left atrium, which increases the pressure in the left atrium. The increased left atrial pressure and decreased right atrial pressure (due to pulmonary resistance) forces blood against the septum primum causing the foramen ovale. This action functionally completes the separation of the heart into two pumps on the right and left sides of the heart. The closed foramen ovale becomes the fossa ovalis. The ductus arteriosis, which connects the pulmonary trunk to the systemic circulation, closes off within one to two days after birth and degenerates as ligamentum arteriosum.
When the umbilical cord is cut, no more blood flows through the umbilical arteries and veins, and they degenerate. The remnant of the umbilical vein becomes the round ligament of the liver and the ductus venosum becomes the ligamentum venosum. The remnant of the umbilical arteries form raised folds in the peritoneum, the medial umbilical folds.
Maternal blood coming into the placenta has an oxygen tension ranging between 95 and 105 mmHg. In the placental intervillous spaces, the tension is estimated to be 30-35 mmHg, while arterial blood returning to the placenta form the fetus has an oxygen tension of 20 mmHg. Despite the relatively low arterial oxygen tension, fetal blood is reasonably well oxygenated because fetal hemoglobin has a much higher affinity for oxygen than adult hemoglobin, reaching a 50% of saturation at an oxygen tension as low as 20 mmHg about.
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