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Before a baby is born, there are a lot of adaptations that we see that allow the baby to take nutrients and oxygen from mom and successfully get those nutrients and oxygen to the different cells that need them in the body. So what I wanted to do is kind of draw out for you in one diagram all the kind of things that we see before birth. These are all the things that are happening while the baby is still in the uterus. Sometimes we say in utero. So before birth, what do we notice? Well, this structure over here, this is our placenta. This is partially mom and partially fetus. So the placenta has mom's blood kind of pooling in this area. And the baby actually sticks its little capillaries inside of that pool of blood. And you can see that the purplish blood is kind of going in, and the reddish blood is coming out. And essentially what I was trying to draw there is that oxygen is getting picked up. So it's actually getting oxygenated. And this blood, as it's kind of reddish, is joining into this blood vessel down here. So this is kind of the smiley part of our face. And this is our umbilical vein. So this umbilical vein is actually going to carry oxygen and blood back towards the liver area. So let me actually just jot that name down, umbilical vein. And this is actually the first of the adaptations I was talking about. So I'm going to make a little list of adaptations over here on the side, just so we can keep track of what they are. And the first one will be the umbilical vein. So once the blood goes into the umbilical vein, it has kind of a branch point. You can see that it can either go to the right or the left. And if it goes to the left, it's going to enter the liver. So if it goes kind of this way, it's going enter the liver. And it's going to take a while for that blood to come out on the other side, because it has to go through all the little capillaries in the liver and then emerge on the other side. But there is a shortcut. So the shortcut-- let me just circle it here. The shortcut is actually going to be right here. So let me just make sure it's very clear what the shortcut is. This is called our ductus venosus. And the ductus venosus is basically going to allow blood to go from the umbilical vein, through it. So it's like a little tube. So it is just like any other blood vessel. It's going to go through it. And on the other side, it hits and meets up with our inferior vena cava. So this is our inferior vena cava. I'll write IVC just for short. And the IVC or the inferior vena cava is a large vein picking up blood from the right leg and also from the left leg. So this is our left leg down here. So the interior vena cava meets up with the blood coming from the umbilical vein, which is very oxygenated. And so this blood I'm going to draw is kind of purplish now, because it's kind of got some oxygen, but it's not as rich as what was coming out initially from the umbilical vein because it mixed in with the IVC. And that blood dumps into the right atrium. So this is our right atrium on this side. And simultaneously, you actually have blood from the superior vena cava, or SVC. This is our head and arm region, draining down this way. And this blood also kind of ends up in the right atrium. So you've got this blood kind of mixing. And now I'm going to draw it as kind of a deeper purple, because it's mixed up blood. Now, the second adaptation, then-- let me just make sure I don't skip out on these. This is the first one. The second one would be the ductus venosus, that I wrote out. Which is, as I said, kind of a shortcut from the umbilical vein over to the inferior vena cava. Now, the blood is in the right atrium. So it has a couple of options. First, it could simply go down into the right ventricle. And some of the blood does that. It just goes right down into the right ventricle. And if goes into the right ventricle, it's going to get squeezed. And once it gets squeezed, it goes into the pulmonary artery. This is my pulmonary artery over here. And we know the pulmonary artery has a branch over to the lungs on both sides. So we've got some blood going to one lung and some blood going to this other lung. But remember, once that blood kind of approaches the lungs, we have to think about what's going on inside of the lungs. So let me draw out what's happening then inside the lungs. You've got these sacs, air sacs, that actually are not full of air. Right? Because when the baby is still inside of the uterus, or when the fetus is in the uterus, it's full of fluid. So you've got these sacs full of fluid. And going past them are little blood vessels. So this is a little blood vessel. And let's say this is an arteriole over here. Now, if it's full of fluid, that means there's not much oxygen. So what ends up happening is that there's a process called hypoxic pulmonary vasoconstriction. And what that means is that the alveolus literally tries to help constrict the arteriole. So the arteriole has some smooth muscle like this. And because there's no oxygen, the alveolus is going to cause that little arteriole to basically contract down. So basically, it looks a little bit more like this. And when it looks like that, what we've essentially done is increased the resistance of that arteriole. And if this is happening millions of times in millions and millions of alveoli, then the entire lung is going to have a lot of resistance. A lot of resistance in the lung at this point. So if that's the case, if there's a lot of resistance, then a few things we have to kind of deduce from that. The first is that if there's a lot of resistance, then the pressure in the pulmonary artery-- remember, this is our pulmonary artery right here, these two. I'll actually draw a little arrow to both of them. The pressure in the pulmonary artery is going to go very high. So these pressures are going to be high. And that's simply because you've got a lot of resistance that you have to try to fight against. So they have a lot of pressure. And if there's a lot of pressure in the pulmonary artery, just think back, and think, well, where did that pulmonary artery come from? It came from the right ventricle. So for there to be forward flow of blood, you better have a lot of pressure in the right ventricle. And then I could take the argument back and say, well, if you have a lot of pressure in the right ventricle, then you must have a lot of pressure in the right atrium. So you have a lot of pressure, basically, on the right side of the heart, because of the fact that you've got a lot of resistance in the lungs. So these pressures, especially the right atrial pressure, starts getting so high that it starts getting higher than the pressure in the left atrium. And so you get a little bit of blood flow that starts going from the right atrium, across that foramen ovale, that allows-- right here-- that allows blood to actually go across it. So this is our foramen ovale. Foramen ovale allows blood to go from one atrium over to the other. And since blood can now go across, you're going to see some of the blood continue down in the right ventricle. But some of the blood will also kind of go across into here. And that's actually quite useful, because at the same time that you have blood going across, you actually don't have too much blood coming back through the pulmonary veins. And the reason for that, again, is because it's hard to get blood flow through the lungs because there's so much resistance there. So you have a little bit of blood kind of coming in through the pulmonary veins. And you get some blood coming from the right atrium. Now, from the left atrium, blood is going to go down into the left ventricle. And on its going to get squeezed around into the aorta. So now you get blood in the aorta. That gets squeezed there or sent there from the aorta-- or from the left ventricle. I apologize. So the left ventricle is squeezing blood down into the aorta. And the aorta is distributing blood all the way down. Now, before I finish off showing you where the aortic blood goes, let's actually make sure I don't forget my list over here. My third adaptation, then, should be the foramen ovale, foramen ovale sending blood from the right atrium to the left atrium. And a fourth adaptation, actually, I've just kind of sketched out, but I haven't talked about yet, is right here. So you actually have this little guy right here. A little connection, a little vessel-- you can think of it as a vessel because blood flows through there-- between the pulmonary artery and the aorta. So this thing right here is called the ductus arteriosus. So the ductus arteriosus allows blood to go from the pulmonary artery to the aorta. And why would blood go in that direction in particular? Well, remember the pulmonary artery, again, has very high pressures. And the high pressures are because of the high resistance in the lungs. So because of those high pressures, blood, of course, goes from high pressure to a place where there's lower pressure usually. And in this case, it's going to go from the pulmonary artery over to the aorta. So it's actually going to flow in this direction. Let me just draw a little arrow. It's going to go flowing in that direction. So ductus arteriosus is another fetal adaptation. So we've got four so far. Ductus arteriosus. And this actually explains, then, why you don't get too much blood coming back through the pulmonary veins. Because a lot of the blood goes into the pulmonary artery trunk, ends up going into the aorta. It actually doesn't even go into the lungs because the resistance is so high. So now let's kind of wrap this up. Let's say blood is now down in the aorta. As I said, it's going to go into the legs. And it's also going to kind of go into these internal iliac arteries. So I've drawn these arteries here. These are the internal iliac arteries. And there are, of course, lots of branches off the internal iliac. But the important branch that I want to point out right now is this one. This branch, this major one that I'm kind of sketching in, this is actually-- we have a name for it. We call this the umbilical artery. So this is actually bringing blood back towards the placenta. Now, why would so much blood go to the placenta? I mean, that's a fair question. Why doesn't it go-- there's a branch here that goes to the bladder. There's a branch that goes to other places. Why is blood going into the placental branch or the umbilical artery? Why so much? Well, it turns out that the placenta-- and this is very clever-- actually has a very low resistance, very low resistance. So just as the lungs have a high resistance and they're kind of making blood divert away from them, the placenta has a low resistance, and it makes blood divert towards it. So you can see now that this is a really ingenious kind of system. We have these five adaptations-- the umbilical vein, the umbilical artery now, we have the ductus venosus, and we have the foramen ovale, and the ductus arteriosus. I don't want to miss out on any of them. So we have five important adaptations here. And this is how blood flows in the fetus.