Oxygen saturation in the umbilical is around 58%.
In healthy adults, oxygenated blood is sent from the left atrium to the left ventricle and then out through the aorta to the arteries in the rest of the body.
Blood then returns through veins to the right atrium and goes into the right ventricle, which pumps this blood to the lungs in order to drop off carbon dioxide and pick up oxygen.
In the fetus, the lungs are not mature enough to complete this last step, so oxygenation happens in the placenta.
Four key adaptations, or structures, make this possible. These are the umbilical veins and arteries in the umbilical cord, the ductus venosus, the foramen ovale, and the ductus arteriosus.
So, imagine that you’re an oxygen rich red blood cell that has to get from the placenta to the fetal tissues.
Blood from the placenta is highly oxygenated, and from the placenta, this blood heads through the umbilical vein, the first adaptation of fetal circulation, toward the liver.
When the umbilical vein reaches the liver, it branches into the left and right umbilical veins.
The left umbilical vein dumps blood into the portal vein, which goes into the liver.
The blood in the portal vein goes out to every lobule of the liver, and it becomes deoxygenated.
This deoxygenated blood enters the hepatic vein, which then drains into the inferior vena cava, which is one of two enormous veins that carry deoxygenated blood from the lower half of the body to the right atrium.
Now, from the left umbilical vein, a vessel called the ductus venosus forms and connects to the inferior vena cava.
This bypasses the liver circulation, and it represents the second adaptation of fetal circulation.
From the inferior vena cava, the oxygenated blood from the placenta mixes with the deoxygenated blood from the lower body.
This mixture of blood is joined by the blood from the hepatic vein before it all flows into the right atrium.
Meanwhile, deoxygenated blood from the upper body flows through the other enormous vein, the superior vena cava, into the right atrium.
Ultimately, the oxygenated blood from the placenta and the deoxygenated blood from the entire fetal body mix together in the right atrium.
Before birth, the fetus’ lungs don’t play a role in gas exchange because there’s no breathing in the womb.
As a result, the arterioles, or tiny arteries, of the lungs are constantly in a low-oxygen environment.
As a result of this, a process called hypoxic pulmonary vasoconstriction happens. More specifically, vasoconstriction, or narrowing, of the pulmonary arteries due to the hypoxic, or low oxygen, conditions takes place.
In other words, the smooth muscle around all of the arterioles in the lung squeezes down when it senses low oxygen levels.
This leads to increased resistance to blood flow in the arterioles, and so the pulmonary artery has really high pressure.
This high pressure causes the right ventricle and the right atrium to remain at relatively high pressures as well.
Therefore, the overall pressure on the right side of the heart is much higher than the pressure on the left side of the heart.
The foramen ovale is an opening made by a tiny flap of heart tissue that acts like a one-way valve.
That opening shunts, or moves blood, from the higher pressure right atrium to the relatively lower pressure left atrium.
So, most of the blood actually bypasses the right ventricle and lungs completely and goes straight to the left atrium and left ventricle before it’s pumped through the aorta to the rest of the body.
Only some of the blood from the right atrium goes down into the right ventricle, and this blood enters the pulmonary artery and heads to the lungs.
For the red blood cells in the high-pressured pulmonary artery, there’s a small blood vessel connecting the pulmonary artery and the aorta called the ductus arteriosus, which is the fourth adaptation of fetal circulation.