Hey everyone, it's Sarah with RegisteredNurseRN.com and today we're going to talk about fetal circulation. And as always, whenever you get done watching this YouTube video, you can access the free quiz that will test you on this content. So let's get started.
To help us understand fetal circulation, we first have to go back and review the normal blood flow of the heart. Like we need to know how the blood actually flows through a heart. that is outside of the uterus that's not depending on a placenta. And we need to know where deoxygenated blood is versus oxygenated blood.
Because the biggest difference between a person's heart whenever they're outside of utero compared to whenever they're inside of utero is that inside of utero, they have these shunts that are shunting, hence pushing away or pulling away blood from the lungs because in utero, the person, the baby is not using their lungs. They're full of fluid but whenever they're born they do use their lungs so those shunts aren't needed anymore because we actually need that blood to go through the lungs. So first let's review the blood flow through the heart. So everything starts on this right side and here in this illustration in the blue this is right side and over here this is our left side. So what's going on on the right side is that all this blood is draining back to your heart.
It is very low in oxygen because that oxygen has went and replenished your tissues and organs so they continue to work. So now it goes on to the right side and its whole goal is to get to the lungs because the lungs are going to replenish that deoxygenated blood with oxygen. does that it's going to enter into the left side which is going to pump it up through your body and replenish your organs and tissues with oxygen and it's just constantly cycling like this so everything we have the inferior vena cava and the superior vena cava and blood is draining back to those that blood is going to go down into the right atrium then it's going to flow down into the right ventricle the right ventricle is going to pump it up through the pulmonary artery. Right now it's still low in oxygen, but once it leaves the pulmonary artery, it's going to hit the lungs where gas exchange is going to happen. So the buildup of waste in your blood like carbon dioxide is going to cross over.
You're going to exhale that out and the oxygen that you've taken in is going to cross over and go in the blood and replenish it. So over here I have the deoxygenated represented in blue. letting you know it's really low in oxygen.
So once it hits that those lungs it's going to flow back through into the pulmonary vein. So now we're into the left side of the heart and it's red it's really high in oxygen. So it's going to go into the left atrium down through the left ventricle which is really strong and it's going to pump that blood up through the aorta and it's going to shoot up through here go to your brain your upper extremities. It's also going to flow down through here through the descending aorta and replenish your lower extremities. So as you can see the right and left side they work beautifully together but they really have their own job.
The right side is going to take deoxygenated blood pump it to the lungs. The left side is going to take that oxygenated blood it just received from the lungs and pump it throughout the body. So the heart and the lungs they work beautifully together.
Now I As you can see in this heart, there are no shunts or little passageways that's allowing blood to flow in different directions, which would help bypass the lungs because we're using the lungs, so the lungs need this blood flow so gas exchange can occur. That is what's different in this fetal heart circulation. Here, our lungs are not working.
They're non-functional. They're full of fluid, but whenever that baby is born, takes their first breath, those lungs... those alveolar sacs, they pop open, baby starts breathing, gas exchange starts to occur.
However, before that even happens, while they're still in utero, the lungs aren't working. So we need a little bit of blood flow to go to those lungs to maintain that lung tissue, but we need majority of it to bypass the lungs. So to do that, in the fetal heart and in its circulation, we have three shunts.
Out of those three shunts, two shunts play a huge role with shunting blood away from the lungs. So for exams you want to remember the names of these shunts, where they're located, and how blood flows through them. So the shunts I want you to remember are the ductus finosus, the foramen ovale and the ductus arteriosus. The ductus venosus is located over here with the liver and we're going to go in depth with these here in a moment.
The foramen ovale is located between the right atrium and the left atrium and the ductus arteriosus helps connect connect the pulmonary artery to the aorta. Now you may be wondering, okay, we're bypassing the lungs so the lungs aren't providing oxygen to the blood, but this baby that's growing in the uterus still needs oxygen in their blood so their brain can develop and their whole body and all those vital organs. So how are they getting oxygen in their blood? Well, they're getting it from the placenta. Now the placenta along with the umbilical cord which attaches to the baby is like the lifeline for that baby whilst developing in the uterus.
If you don't have a healthy placenta, there's going to be issues with the baby growing and developing properly along with major complications that can happen. So the placenta, what it does is it embeds itself in the uterus. And when it does this, it in a sense connects with mom's circulation because this placenta, what it's going to do is it's going to give the baby oxygen and nutrients that it pulls from mom's circulation, send it through the umbilical cord, go to baby.
And then baby's going to use it up, produce its own waste, and it needs to get rid of it. So the baby... will send it back through the umbilical cord to the placenta which will go into mom's circulation and mom will clear it so think of the placenta as like the temporary lungs for the baby until they're actually able to use their own set of lungs now for exams what you want to know is really about this umbilical cord specifically about the umbilical vein and the umbilical artery like how many there are of each and what type of blood they carry So with this umbilical cord there is usually in most babies there is only one umbilical vein and what this umbilical vein does is it carries oxygenated blood so it's going to carry it away from the placenta it's illustrated here in this red and you can see the arrows and it's going to take it away from the placenta because it's rich in oxygen and nutrients and send it to the baby.
Now there are two umbilical arteries and they carry deoxygenated blood. So blood that's really low in oxygen. So here it's represented in blue. We have the arrows going back to the placenta.
So the baby has used up that nice nutrients and oxygenated blood that the mom sent it, but now it has waste in it and it needs to get reoxygenated. So it's going to leave via these two umbilical arteries and send it back to the placenta. So now let me walk you through fetal circulation. circulation of a baby who is in utero and then follow that up with how that fetal circulation changes once they are born and they start breathing on their own. So we're going to start here up at this placenta and specifically we're going to start at the umbilical vein.
How many umbilical veins are there? One. And what kind of blood does it carry? Oxygenated. So it has nutrients and oxygen in it.
So it has pulled from mom's circulation that oxygen and nutrients and it's going to flow down through. this vessel and then it's going to hit the liver. Now the liver is not fully functional yet but it does have some blood flow so some of that blood is going to go through the hepatic portal vein. The majority of it is going to be shunted to the inferior vena cava. So this is our first shunt the ductus venosus.
So the ductus venosus is going to help shunt this rich oxygenated nutrient rich blood and it wants to get it to the heart so it's going to enter in to the inferior vena cava now also in the inferior vena cava just like over here with a normal heart blood flow it's blood that's draining back from the baby that is low in oxygen that's a deoxygenated so this blood is going to flow up and it's going to enter into the right atrium just like how it did over here now there's some really cool things going on in this right atrium First of all, most of this rich oxygenated blood is actually going to flow over here to this left side via a shunt, which is our second shunt, called the foramen ovale. The foramen ovale is going to connect your right atrium to your left atrium. Because remember, what did we say over here was the whole goal of this right side? To get deoxygenated blood to the right side. lungs but over here our lungs aren't working so we need to take this fresh blood with oxygen that we just received from mom and we need to get it to this left side because the left side is going to get it to the body we don't need it to go through this right side and hit the lungs so The majority of this fresh, rich oxygenated blood is going to go and flow over here into the left atrium.
It's going to go down into the left ventricle, up through the aorta and replenish the body. that is occurring in this right atrium is the flow of blood draining back to the body that is deoxygenated. So you have the superior vena cava flowing in this deoxygenated blood. The majority of this blood is actually going to go down into this right ventricle because the body needs to get this deoxygenated blood back to this placenta.
And how it's going to do this is it needs it to go here on this side. So the blood is going to flow down here, hit the right ventricle. Now also, some of this rich oxygenated blood that came from this umbilical vein is also going to flow down in here as well. So the blood is going to be mixed with oxygenated and deoxygenated blood.
And it's going to be pumped up through the pulmonary artery. Now normally this pulmonary artery would have sent it to the lungs. Well, it'll send a little bit to the lungs. to maintain the lung tissue but most of it is going to actually be shunted away and this is where our third shunt is this is the ductus arteriosus so it's going to be shunted from the pulmonary artery into the aorta so that's what our ductus arteriosus does it connects our pulmonary artery to our aorta now you may be wondering how is this able to happen how are these structures able to allow blood to be shunted here and to cross from right atrium to left atrium well there's a pressure difference let me go over this with you real fast in these lungs that are non-functional the resistance is really high because there's fluid in there because it's really high it's making the pressure in this right side a lot higher than the left side so because we have that pressure difference This allows oxygenated blood to easily flow through this foramen ovale and hit this left side.
Also, it allows the blood to just flow up through this ductus arteriosus into the aorta. Because it's easier for blood to flow from a higher resistance to a lower resistance. Now, keep that in mind because after birth, those pressures are going to change.
And that is what really affects our shunts and helps them. still close off. So just keep that in mind.
So majority of this mixed blood is flowing down through this aorta, specifically the descending aorta. Now some of that blood is going to go down to the lower extremities and help those out because it's mixed. It has some oxygen in it, but this descending aorta is going to branch off and it has different branches.
And we have the internal iliac artery, which will connect. to our umbilical arteries and again, what do the umbilical arteries do? There's two of them.
They carry the deoxygenated blood So we're gonna take that blood and we're gonna get it back to mama who is going to Remove that waste and help replenish it. So this cycle will just keep going over and over So now let's talk about how this changes after birth So a baby is born and they start breathing on their own. What's one?
one of the first things they do after the baby is born. They cut the umbilical cord. So when we cut the umbilical cord, we are stopping this whole transfer of blood flow back and forth.
So especially with the umbilical vein, we do not need the ductus venosus. So it disappears. Now remember the resistance thing I was talking about earlier with the lungs.
So before a baby's breathing on their own, they have fluid in their lungs, the resistance is really high. in here, which is making the resistance on the right side a lot higher than compared to the left side of the heart. But once baby starts breathing, they clear all that fluid out of their lungs, those sacs pop open, the resistance drops. So whenever that drops, that's going to change the pressure over here on the right side of the heart.
It's actually going to become a lot lower on the right side compared to the left side. So now we don't need that shunting of the blood. It's not really going to happen.
to happen because we've changed the pressure. So this is going to help the foramen ovale close. So you won't have this connection anymore between your right atrium and left atrium, because now we need blood to go and do its job like how it does on the right side.
We need to majority up through this pulmonary artery and hit those lungs. Now in some people, this foramen ovale doesn't close. And we call this a patent foramen ovale, a PFO.
And I have a whole series on congenital heart defects if you want to check out. that video that we'll talk about this along with the patent ductus arteriosus so that closes then we have the ductus arteriosus and this closes we have the pressure changes not going to have the high pressure here pulmonary artery allowing it to flow up here into the aorta but we've also removed this placenta the placenta produces prostaglandins and this vessel is really sensitive to prostaglandins So when we cut that out, we decrease the prostaglandins, that's going to help close this ductus arteriosus, plus the baby's breathing on their own, oxygen levels are up, and so we don't need that structure anymore, and it will seal off. Okay, so that wraps up this review over fetal circulation, and don't forget to access the free quiz that will test you on this content.