Fetal circulation

Definitions & Key takeaways

Fetal circulation is different from adult circulation because the fetus' blood doesn't mix with the mother's blood. The placenta acts as a filter, so the baby receives nutrients and oxygen from the mother, and sends its metabolic wastes into the mother's circulation for elimination. The fetal circulatory system has some special adaptations, such as the foramen ovale, an opening between fetal heart atria; the ductus arteriosus, a small vessels that shunts blood from the pulmonary artery to the aorta; the ductus venosus which shunts blood from the umbilical vein to the inferior vena cava; the umbilical arteries which carry deoxygenated blood from fetal circulation to the placenta; and the umbilical vein, which returns oxygenated blood from the placenta to the fetus , and the ductus venosus.

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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.
These are the umbilical veins and arteries in the umbilical cord, the ductus venosus, the foramen ovale, and the ductus arteriosus.
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.
Meanwhile, deoxygenated blood from the upper body flows through the other enormous vein, the superior vena cava, 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.
The ductus arteriosus shunts blood from the higher pressure pulmonary artery to the relatively lower pressure aorta. This opening is kept open in part by a chemical called prostaglandin that is made by the placenta during fetal development.The aorta then sends blood to the entire body through its various branches.
Now as the aorta goes down it splits into the right and left common iliac arteries, and those both split into internal and external iliac arteries.
Each of these internal iliac arteries give rise to an umbilical artery. These two umbilical arteries travel alongside the umbilical vein and bring deoxygenated blood back to the placenta, where it drops off carbon dioxide and picks up oxygen.
The cycle repeats itself this way until birth.Now at birth, everything changes. As soon the baby is born, the baby begins to cry.
Air flows into the lungs for the first time, and it replaces the water that previously filled the alveoli. As oxygen levels in the lungs and the blood rise, the hypoxic vasoconstriction goes away, and the smooth muscles around the pulmonary arterioles relax.
The pulmonary resistance falls and so does the pressure in the pulmonary artery. Blood can easily flow into the lungs and all of a sudden, the pressure on the right side of the heart is much lower than pressure on the left side of the heart.
The flap of tissue that forms the foramen ovale, gets pushed in the opposite direction, and like a one way valve, it doesn’t allow blood to go from the left atrium to the right atrium.
It basically shuts off the foramen ovale, and over time the tissue seals shut permanently - forming the fossa ovalis. All the blood in the right atrium goes directly into the right ventricle where it gets pumped into the pulmonary artery and the lungs.
The drop in pulmonary artery pressure, also means that there is a temporary reversal in blood flow through the ductus arteriosus.
Blood from the aorta now shunts into the pulmonary artery. This is only temporary though because the higher oxygen levels in the blood cause the smooth muscles in the walls of the ductus arteriosus to constrict.
After delivery, the placenta is separated from the baby, and that means that prostaglandin levels fall, which also contributes to closing of the ductus arteriosus.
Within a few hours after birth, the ductus arteriosus begins closing up, the vessel flattens out and eventually seals completely, forming the ligamentum arteriosum.Next there are the two umbilical arteries and umbilical vein which are surrounded by a yellow, gooey substance called Wharton’s jelly in the umbilical cord.
When exposed to the cold world outside the womb, Wharton’s jelly shrinks down and squeezes the umbilical blood vessels, causing them to wither.
The smooth muscle in the walls of the umbilical arteries constricts and they flatten out like strands of pasta. Within a few months, the umbilical arteries are mostly gone, with just a central portion that remains functioning as the superior vesical arteries which supply blood to either side of the bladder.
Meanwhile, the blood in the umbilical vein and the ductus venosus starts clotting up over a period of days and the blood flow eventually stops.
The remnants of the umbilical vein forms round ligament of the liver and the remnants of the ductus venosus forms the ligamentum venosum of the liver.
All right, as a quick recap… During development, the fetal circulatory system contains four key adaptations - these are the foramen ovale, the ductus arteriosus, the umbilical arteries and vein, and the ductus venosus.
Once the baby is born and takes a breath, lots of changes take place. The foramen ovale becomes the fossa ovalis, the ductus arteriosus becomes the ligamentum arteriosum, the umbilical arteries become the superior vesical arteries, the umbilical vein becomes the round ligament of the liver, and the ductus venosus becomes the ligamentum venosum.