In the adult, oxygenated blood is sent from the left atrium to the left ventricle and then out the aorta to arteries in the rest of the body. Blood then returns through veins to the right atrium and goes into the right ventricle, which pumps it to the lungs in order to drop off carbon dioxide and pick up oxygen.
In the fetus, the lungs are not mature enough to do that, so oxygenation happens in the placenta, and four key adaptations or structures make this possible.
So imagine 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 blood, so let’s color that red.
From the placenta, blood heads through the umbilical vein, the first adaptation of fetal circulation, that carries oxygenated blood toward the liver.
When the umbilical vein reaches the liver, it dumps blood into the portal vein. The blood in the portal vein goes out to every lobule of the liver, and becomes deoxygenated so we’ll color it blue, although in reality it’s more of a dark, dark red color.
This deoxygenated blood enters the hepatic vein, which then drains into the inferior vena cava, which is one of two enormous veins that carries deoxygenated blood from the lower half of the body to the right atrium.
Now, from the umbilical vein, a vessel called the ductus venosus forms and connects to the inferior vena cava. This bypasses the liver circulation, and represents the second adaptation of fetal circulation..
From there, the red oxygenated blood from the placenta mixes with the blue deoxygenated blood from the lower body, so red and blue make purple, and that purple blood is joined by the blood from the hepatic vein before it all flows into the right atrium.
So ultimately oxygenated blood from the placenta and deoxygenated blood from the entire fetal body, all gets mixed up in the right atrium.
Now 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 tiny arteries - called arterioles - of the lungs are constantly in a low oxygen environment.
As a result, there is a process called hypoxic pulmonary vasoconstriction that takes place - that’s where there’s vasoconstriction or narrowing of the pulmonary arteries due to the hypoxic or low oxygen conditions.
In other words, the smooth muscle around all of the arterioles in the lung squeeze down when they sense low oxygen levels. This leads to increased resistance to blood flow in the arterioles that makes the pulmonary artery have really high-pressure.
That high pressure causes the right ventricle and the right atrium to all remain at relatively high pressure as well. And overall pressure on the right side of the heart is much higher than pressure on the left side of the heart.
Now, in the fetal heart, there’s an opening between the atria called the foramen ovale, which is the third adaptation of fetal circulation. 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 and gets pumped out the aorta to the rest of the body.
Only some of the blood from the right atrium goes down into the right ventricle, and enters the pulmonary artery, heading for 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 to fetal circulation.