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Watch how the blood flows through the baby's circulation and compare it to what happens in the fetus. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.
Video transcript
We've talked about fetal circulation, and I've talked about all the different interesting adaptations that the fetus has to make sure it can adjust to life within the uterus, within mom. But when the baby comes out-- let's say the baby is just delivered-- there's got be a lot of changes that happen. In fact, these adaptations, each of them plays a role in the first few minutes, hours, days of life. And so what I wanted to do is go through all the adaptations, think through them, and see what's happening actually after birth. So we know what happens before birth and how the baby adjusts there, but how does this now translate into what's going to happen after birth and what the baby has to do now that it's separated from mom and breathing on its own? And the first two things I want to point out are the idea of-- what are the big things that are changing? And one big thing is, of course, that the placenta, which the baby's been using for 40 weeks, or nine months or so, is no longer around. The placenta is removed from the baby's circulation. We're going to cut it away. And the second big thing that's going to happen is that the lungs get used to bring in air for the first time. So the lungs take in air. So these are the two huge things that are going to change. And these two things are going to end up affecting a whole lot of other things, as well. So let's get started. Let's see what happens when the placenta gets removed and when the lungs take in air. Let's start with the placenta. So let's say that you decide that the baby is now delivered, and you want to cut the cord, cut the umbilical cord, and put an umbilical clamp right there. And this is often done. You'll see this done in movies, or if you've ever gone to a delivery, you'll see this done pretty routinely. So this is a little umbilical clamp, and it's clamping the cord. And if you're ever worried about whether that hurts the baby or the mother, it doesn't. Because the umbilical cord does not have nerves. So that's kind of the first interesting thing about it. But this stuff, remember-- this pale yellowish stuff that's kind of jelly-like-- we call this Wharton's jelly. Wharton's jelly. And one of the things that I always thought was really cool about Wharton's jelly is that it's a really interesting Mother Nature-type idea, that the Wharton's jelly starts contracting. It gets kind of tighter around the three vessels-- the two eyes and the smiley face that I've drawn here, which are the two umbilical arteries in the vein. The Wharton's jelly starts squeezing around those vessels as soon as the temperature falls. So temperature falls-- and remember, the temperature in the mom is going to be much warmer than it is outside in the delivery room, so immediately that Wharton's jelly is exposed to cold air. And when the temperature falls, the Wharton's jelly starts to contract, causes contraction. And of course, that's going to squeeze down on all the vessels inside. It's going to basically clamp down on them. And so it's almost like we have this man-made clamp that I drew in orange, but the Wharton's jelly is kind of a natural clamp that we already have. So we're taking this very low-resistance placenta-- remember, it used to be very low-resistance, a lot of blood liked to flow in that direction-- and creating really high-resistance. So this is the first big change, is that the placenta gets removed and you go from low-resistance to high-resistance. So that's a key idea. Now as a result of the high-resistance-- remember, there used to be blood flowing through the umbilical vein, but now in the first few days, there's really no blood flowing through here. All the blood starts clotting off. And that's true even of the ductus venosus. You get some blood clots in there. So you don't really have any flow anymore, and in the first few days, you really completely lose any flow through those things. So this becomes non-used, or unused, over the first few days of life. Now you still have blood flowing from the portal vein into the liver, and you still have blood going up the inferior vena cava, and this is all deoxygenated blood, so that is still the same as before. And this deoxygenated blood now has no new fresh oxygen to mix with. So I'm not going to color it purple. I'm going to leave it the same blue color. So deoxygenated blood comes up from the legs and it comes down from the head and the arms, from the superior vena cava. And now all this deoxygenated blood fills in the right atrium, and some of that blood is going to now go into the right ventricle, so let's color that in blue. And it's going to get squeezed out into the pulmonary arteries from the right ventricle, so let me color that in the same deoxygenated blue color. And this is headed toward the lungs. Now in the lungs, what was happening? Well, initially, remember we had these little alveoli. And they're full of fluid. And that fluid now is going to get replaced by air. So air is going to push the fluid out. Air is going to push all that fluid out. And what's on the outside? Well, we've got little capillaries. So we've got these capillaries, and the fluid will enter the capillary. But remember, right before the capillary is the arteriole. Let me actually sketch it a little bit smaller, the arteriole. Because it used to be very constricted. Remember, there was that hypoxic pulmonary vasoconstriction. But now that you have air in there, the oxygen levels are rising in the alveolus. And what that's going to do is that's going to send a signal over to the arteriole-- this is our arteriole-- to say, hey, it's time to open up now. It's time to finally dilate. So this arteriole is excited. It's never really been very dilated before in its life. So it finally says, yay, it's my chance. So it dilates. It dilates like this. And it's nice and plump and big. And when it gets big, what does that really mean? It means that the resistance has fallen. Resistance has fallen. So remember, the lungs used to have high resistance. And now, millions of alveoli are causing the arterioles to open up and resistance falls. And this happens, of course, on both sides. So on the left lung and the right lung, the resistance is falling. And that deoxygenated blood now can flow in. Because initially, it wasn't really wanting to flow in because the pressures had to be so great. But now the pulmonary artery pressures are falling. It's easier to actually get the blood into the lungs. And that means, of course, the right ventricular pressures are falling. And the right atrial pressures are falling. So the entire right side of the heart now is working under lower pressures because the resistance in the lungs has gone down. And now the resistance in the lungs going down, that means that more blood is going to go in, and if it goes in, it's going to go into all the little thousands of capillaries and it's going to get oxygenated. And those capillaries are going to send all that blood back and it's going to flow into the left atrium. So you have all this fantastic oxygenated blood coming in from both sides, coming into those pulmonary veins. So now tons of oxygenated blood is dumping into the left atrium, which is different than before, because you didn't have much flow through the lung. So now you've got lots of blood kind of flowing in here. And at the same time, the pressures on the right side have fallen. So if pressures on the right side have fallen, think about what's happening to our foramen ovale. Before, blood was actually kind of gushing through there. But now, because the pressures on the right side are so low, this little flap of tissue, like a little valve, closes off. And so now you can actually see that this flap of tissue will do this. It'll close off. Because you basically have more pressure on the left side than the right side, and it pushes that flap of tissue over. And now the foramen ovale is basically closed. And this happens, actually, in the first few minutes-- first few minutes after a baby is out of the mom, you actually see this foramen ovale close, which is amazing. Now blood continues to go down, it likes to go into the left ventricle. So it's going to go down here and get squeezed into the aorta. So let me show-- now oxygenated blood for the first time kind of getting into the aorta this way. And then you have the question of the ductus arteriosus. Remember, initially the reason that blood was moving from the pulmonary artery into the aorta was because the pressures in the pulmonary artery were so high. But now the pressures are pretty low, the pressures are much lower. If anything, you would actually have flow going this way because the aortic pressures are higher than what the pulmonary pressures are now. But it turns out, interestingly, that in the first few hours of life, you actually have some constriction of the muscles in that ductus arteriosus. So that ductus arteriosus has smooth muscle in the walls. And those smooth muscles are going to sense that now oxygen levels are high. They're going to sense the increase in oxygen levels in the blood. And they're going to start getting twitchy, they're going to want to start constricting. The other thing that the ductus arteriosus senses is that the placenta is removed. How would you sense something like that? And how would the ductus-- which is over here-- how would it sense that the placenta-- which is over here-- how would it know that it's been removed? Well, it turns out the placenta actually makes a little chemical called prostaglandin. And when prostaglandin levels fall, when prostaglandin levels go down, then the ductus arteriosus also is more willing or able to close down. So those little muscles inside of the ductus arteriosus-- remember, it's like a little artery, in a sense. It's got smooth muscle around it. Those muscles are going to constrict, they're going to tighten down when the oxygen levels go up and when the prostaglandin levels go down. It's going to sense that. And so it's going to know that hey, it's time for me to close up shop and tighten down. And over time-- and I'll say over a course of hours-- this is going to happen. So let me actually just jot down the time frame for you. So over the course of a few hours, the beginning of constriction will happen. So over time, this will actually get kind of tighter and tighter and tighter. Let me sketch it out, getting smaller and smaller and smaller. You actually have on the inside of it maybe a tiny little opening, and then over time, a smaller opening, and over time, no opening at all. So that's going to happen at the beginning of the first few hours of life. Now, following the blood all the way down, you actually have aortic blood with oxygen flowing down here into the, let's say, the right leg and into the left leg, over here. And there are these branches, these big branches, called the internal iliac branches, and off of them, where the umbilical arteries, right? The umbilical arteries where branches off of the internal iliac. And what's going to happen is that you're going to still get blood flowing to other branches off the internal iliac, like this little branch might go to the bladder. But that last little bit, really, there will be no flow through there because the resistance is so darn high. Because the resistance over here is so darn high, no blood is going to want to go in that direction. In addition, the umbilical arteries, just like the ductus arteriosus, have smooth muscle in them. And so that smooth muscle is going to respond to the very high levels of oxygen that, for the first time, these arteries are seeing, and low prostaglandins that are kind of circulating. And they're also going to kind of start constricting. So just as the ductus arteriosus started constricting, these arteries also start constricting. They get tighter and tighter and tighter until really there's almost no space in the middle left. And that's how I'm going to draw it fizzling out. So initially, they get kind of more narrow and they get even more narrow as the muscles in the walls tighten and tighten and tighten. And they finally get something like this. And you still have blood, of course, going to other branches, which is what I've drawn here. But that last little bit going just to the umbilicus, that part is going to constrict down. And this process happens over the course of a few hours. So now you have it. You have all the five adaptations and how things change over the course of minutes, hours, and days. And of course, it's not exact and each baby is different, but these amazing changes are happening soon after birth.