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Learn how the needs of the fetus are met by the placenta, which is a special organ that belongs to both the mother and the fetus! Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.
Video transcript
- [Voiceover] In this picture we've got a pregnant mom and a little fetus inside of her uterus. That's the name of this little chamber that the fetus is hanging out in. This uterus is an organ and women have this organ. It's a very, very strong organ, very muscular organ. In fact, if you've ever gone to a delivery and you've heard people chant, "Push, push, push," they're literally kind of encouraging mom to push down on this uterus and help the baby come out into the world. So this uterus is the room in which the baby lives. But the environment in which the baby lives is actually very different as well from how we live outside, in the sense that there's amniotic fluid in this space, right, and the baby is actually swimming in this fluid, for months at a time. For nine months, baby is swimming in amniotic fluid and that brings up some obvious questions. How is baby able to breathe or eat? How does it take care of its needs? Let's think about what those needs are. Let's enumerate them. One of them is oxygen. How does a baby take in oxygen if it's not getting it from the lungs? The lungs are actually full of amniotic fluid, by the way. How is it able to get nutrients? Things like sugar, glucose, that it needs for energy. I'll put example, glucose. Let's think about waste. How does it get rid of things like carbon dioxide? For us, it's very easy, we just kind of exhale or take a breath out. But how does a baby do that if I just said that the lungs are full of fluid? How does it get rid of waste products like carbon dioxide? It turns out all of these kind of issues are obviously very important, and the mom and the baby work together a solution. This is the first chance for mom and baby to really work together on something. The solution is something called the placenta. You may have heard of the placenta. The baby actually sends a couple of vessels up to the placenta. These are little umbilical arteries. It also sends up an umbilical vein to the placenta. The vein brings blood back from the placenta. In all these vessels, all three of these, make what we call the umbilical cord. You may have heard of the umbilical cord and now you know what's inside of it. The umbilical cord actually, if you were to cut it, I'm gonna show you what it would look like if you were to cut across it. It's actually an easy thing to remember because it has a big, floppy little vessel right here. That's the umbilical vein. It's got oxygen, that's why I'm drawing it as red even though it's a vein because it's coming back to the heart, that's why we call it a vein. And you've got umbilical arteries. These two actually don't have too much oxygen. That's why I'm drawing them in blue, just to show that they're low in oxygen. They're called umbilical arteries because they're going away from the heart. The umbilical arteries and the umbilical vein look like this, actually they're kind of they're not just freely floating out there. They're actually encased in this jelly, and the jelly is called Wharton's jelly. I'm actually just gonna redraw this, just to show you how I remember this. Sometimes it' hard to remember if there's one artery or two arteries. I think of the mouth as a vein, or a V. And I think of the eyes as two little A's. That's my little trick for remembering that there are two umbilical arteries and one umbilical vein, and that they're all encased in the Wharton's jelly, and this is the umbilical cord. This is the overall picture, now I wanna go into some detail to really get us to understand how the placenta works because it honestly is one of the most interesting organs in the sense that it allows us to do things that we simply cannot do today, but we could do for many, many months at a time. I'm gonna invert the way that I drew this. I drew this one way above and I think you'll see how I flipped it around here. This'll be the uterine wall. Again, remember the uterine wall is a nice, strong, muscular wall. Let me label it so it's clear. This is uterine, I'll just call it uterus, the wall of the uterus anyway. On the insides you've got, actually I'm working from the outside in. Let me draw some of mom's blood vessels. She's got some blood vessels coming up like this, this is a little uterine artery. This, of course, gives oxygen to the uterus. We said it's a big muscle so it definitely needs blood vessels, like arteries and veins. I'll draw a couple of them. I'll draw another uterine artery over here, and I'll draw one third one over here. Of course, they are many more than three but I just wanna give you the concept that there are some uterine arteries. There are also some uterine veins. Let me draw some uterine veins as well. I'll draw one over here. These blue ones are the uterine veins, I'll label this one over here. What they're doing is they're actually going through a layer of tissue. Let me draw in a layer of tissue for you so you see exactly what I mean. There's a layer of tissue here, they're going through it. This layer of tissue is mom's tissue. I'm gonna label it mom's tissue, or I'll just call it mom's. It looks a little bit, you have to stretch your imagination, but it looks a little bit like a plate. This entire thing that I'm shading in in red is one of the parts of the placenta. So we've already started drawing the placenta. Actually, let me switch colors. This part is known as the basel plate. If you're talking about the basel plate you're talking about this bit of tissue here, with the blood vessels, the uterine arteries, and the uterine veins. Just to be clear, it belongs to mom. This is mom's cells. Now we get into some really fun stuff. All these uterine arteries, they squirt out blood. They don't go into any other vessel. They just squirt out blood into an open area. Let's say there's a red blood cell here, this red blood cell literally could touch, physically touch a red blood cell over here. They literally could come up, and meet, and maybe shake hands. They could then go down the veins. This is really, really different from how we usually think about blood cells traveling always within a vessel. Cuz here it's kind of a pool. You have this circulation that happens because you have these red blood cells, as I've drawn them, kind of going up, into the middle, then circulating into the veins. You have this interesting circulation where you basically have blood coming up through the arteries and down into the veins. The way I think about this, and you can maybe choose your own kind of analogy. But for me, I think of it a little bit like a hot tub. The uterine arteries are like the jets of water, coming up. The uterine veins are literally like the drains in the hot tub. It becomes almost like a swirling pool of red blood cells. Importantly, these red blood cells have with them oxygen, they have nutrients, so they have a lot of the stuff that our fetus is interested in getting access to. Let's switch gears, let's think about our fetus. Our fetus has got, we said a couple of umbilical arteries coming down. I also said that they're coming down inside of this Wharton's jelly, this umbilical cord of Wharton's jelly. Actually what happens is that the vessels actually branch out and they split up. The uterine arteries, form lots and lots and lots of little branches. I'm gonna draw them in for you. Let's say that the branches go all over the place. They go like this. Let's draw five or six branches over here. Let's draw some branches down on this side too. We've got lots of branches off the uterine arteries. They want access to the oxygen and nutrients. Guess what they do? They just kind of push in, in fact, you have little cells here. I'm gonna draw some pushing in right here. You got little cells that I'm drawing in white, called trophoblast. They literally do this. They just kind of push in. These white cells are trophoblast cells. They're good at invading. You could think of them as invading or if you wanna be less aggressive, you could think of them as being just curious. These little fetus cells, called trophoblasts, start pushing in. And within the trophoblast is a little blood vessel. This blood vessel now is in very close contact with mom's red blood cells, you've got red blood cells coming up alongside of it. Now what's gonna happen through diffusion, is that oxygen is gonna go into the fetus' blood cells. So oxygen is going this way, as well as nutrients. Oxygen, nutrients go inside, then you have carbon dioxide leaving. So carbon dioxide actually goes into the pool of blood. It's a really, really cool thing that the red blood cells aren't touching, the mom's red blood cells are not touching baby's red blood cells cuz, of course, baby's red blood cells are within blood vessels, these are within blood vessels. These things right here are not open, they're closed vessels. Once that exchange happens, they continue to stay closed, and you get a little capillary coming back. You've got capillaries coming back from every part of this. All of these have been doing the same thing. Going inside of this pool of blood. It actually gets a little bit more complicated than the way I've drawn it, in fact, a more accurate drawing might be with more branching. You might have something like this, where it really has lots of surface area. It comes back around, and you have the blue vessels going into each one of these finger-like projections. You have a lot of gas exchange happening. This would be maybe a better way of drawing it but the concept is the same. You basically have all this wonderful diffusion happening. As a result you actually have nice, rich oxygenated blood returning to the baby. This is finally allowing the baby to have access to oxygen and nutrients and get rid of its waste, its carbon dioxide. Each one of these is actually doing the same thing with red or oxygenated blood coming back. Instead of having two vessels, it all dumps into one vessel down the middle. This vessel, as we already know, is called the umbilical vein. This is actually the umbilical vein. If we call the bottom one the basel plate, then, of course, you're guessing that there must be some name for this. This, of course, has its name called the chorionic plate. This one, you can already tell, this has all the cells from the fetus, so this belongs to the fetus. You've got some tissue that belongs to the fetus, that would be the chorionic plate and all of its contents. You've got some tissue that belongs to mom, that's the basel plate. Then you've got some shared tissue, or space, in here. All these little projections belong to the baby. But the actual pool, this pool of blood that's circulating. That pool of blood belongs to mom. So really, the placenta is a combination of all of these things. I think of the placenta then as the first time that mom and baby work together for a common purpose.