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学习胎盘这一同属于妈妈和宝宝的特殊器官如何满足婴儿所需。里希是一名儿科传染病医师,在可汗学院工作。 Rishi Desai 创建




so 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 and this uterus is an organ and women have this organ and 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 kind of chant push push push they're literally kind of you know encouraging mom to push down on this uterus and help the baby come out into the world so this uterus is is the room in which the baby lives but the environment in which the baby lives is actually very different as well from from how we live outside in the sense that there's amniotic fluid in this space right amniotic fluid and the baby is actually swimming in this fluid for months at a time so for nine months baby is swimming 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 so let's think about what those needs are you know let's let's enumerate them one of them is oxygen so you know how does the baby take in oxygen if it's not getting it from the lungs and the lungs are actually full of amniotic fluid by the way how is it able to get nutrients you know things like sugar or glucose that it needs for energy I'll put an example glucose how about let's think about waste how does it get rid of things like carbon dioxide you know for us it's very easy we just kind of exhale or take a 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 and it turns out all of these kinds of issues are obviously very important and the mom and the baby kind of work together a solution and this is the first kind of chance for mom and baby to really work together on something and the solution is something called the placenta the placenta and you may have heard of the placenta and the baby actually sends a couple of vessels up to the placenta and these are little umbilical arteries and also sends up an umbilical vein din the placenta and the vein brings blood back from the placenta and all these vessels all three of these kind of make what we call the umbilical cord so 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 just gonna show you what it would look like if you were to cut across it it's actually kind of an easy thing to remember because it has a big floppy little vessel right here and that's the umbilical vein that's the umbilical vein and it's got oxygen and that's why I'm drawing it as red even though it's a vein because it's coming back to the heart so that's why we call it a vein and you've got umbilical arteries and 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 but they're called umbilical arteries because they're going away from the heart so the umbilical arteries in the umbilical vein look like this and 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 not you're just gonna redraw this just to show you how I remember this because sometimes it's hard to remember if there are one if there's one artery or two arteries and I kind of think of the mouth as a vein and or a V and I think of the eyes as two little A's so that's my little trick for remembering that there are two umbilical arteries in one umbilical vein and that they're all encased in the Wharton's jelly and this is the umbilical cord right so this is kind of the overall picture but now I want to kind of go into some detail to really get us to understand how the placenta works because it's and 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 so I'm gonna invert the way that I drew this so I drew this kind of one way above and I think you'll see how I flipped it here this'll be the uterine wall and again the member of the uterine wall is kind of a nice strong muscular wall and let me label it so it's clear this is uterine neurology called uterus and the wall of the uterus anyway and on the insides you've got actually working from the outside in let me draw some of mom's blood vessels so she's got some blood vessels coming up like this and this is a little uterine artery and 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 and I'll draw a couple of them let's draw another uterine artery over here and I'll draw one third one over here and of course there are many more than three but I just want to kind of give you the concept that there are some uterine arteries and there also some uterine veins let me draw some uterine veins as well and I'll draw one over here so these blue ones are the uterine veins I'll label this one over here so what what they're doing is they're actually going through a layer of tissues so let me draw in a layer of tissue for you so you see exactly what I mean there's a layer of tissue here and they're kind of going through it and this layer of tissue is Mom's tissue so actually I'm gonna label it mom's tissue or I'll just call it moms and it looks a little bit you have to stretch your imagination but it looks a little bit like a plate so this entire thing that I'm shading in in red is one of the parts of the placenta so we've already kind of started drawing the placenta and this part is known actually my switch colors this part is known as the basal plate so if they're talking about the basal plate you're talking about this this bit of tissue here with the blood vessels the uterine arteries in the uterine veins and just to be kind of clear it belongs to mom so this is moms cells and now we get into some really fun stuff so this all uterine arteries they kind of squirt out blood and they don't go into you know any other vessel they actually just kind of squirt out blood into an open area so let's say there is a red blood cell here this red blood cell literally could touch physically touch a red blood cell over here they literally just could come up and and meet and maybe shake hands and then they could then go down the veins so this is really really different from how we usually think about blood cells traveling always within a vessel because here it's kind of a pool and you have this kind of circulation that happens because you have these red blood cells kind of as I've drawn them kind of going up into the middle and then circulating into the veins so you have this interesting circulation where you basically have blood coming up through the arteries and down into the veins and 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 and the uterine arteries are like the Jets of water kind of coming up and the blue veins the uterine veins are literally like the drains in the hot tub so it becomes almost like a swirling pool of red blood cells now 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 so let's switch gears let's think about our fetus our fetus has got we set a couple of umbilical arteries coming down and I also said that they're coming down inside of this Wharton's jelly right this umbilical cord of Wharton's jelly and actually what happens is that the the vessels actually branch out and they split up so the uterine arteries they form lots and lots and lots of little branches and I'm gonna draw them in for you and let's say that the branches go kind of all over the place and say they go like this let's draw five or six branches over here and let's draw some branches down on this side too we've got lots of branches off the uterine arteries and they want access to the oxygen and nutrients so 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've got little cells and I'm drawing in white called trophoblast and literally kind of do this they just kind of push in so these white cells are trophoblast cells and they're good at invading you could think of him as invading or or if you want to be less aggressive you could think of them as being just curious so these little fetus cells call trophoblast start pushing in and within the trophoblast is a little blood vessel and this blood vessel now is in very close contact with mom's red blood cells right you've got red blood cells coming up alongside of it and now what's going to happen through diffusion is that oxygen is gonna go into the fetuses blood cells so oxygen is going this way as well as nutrients so oxygen nutrients go inside and then you have carbon dioxide leaving so carbon dioxide actually goes into the pool of blood so it's a really really kind of cool thing that the the red blood cells aren't touching the moms red blood cells are not touching baby's red blood cells because of course baby's red blood cells are within blood vessels right these are within blood vessels these things right here are not open they're closed vessels and once that exchange happens they continue to stay closed and you get like a little capillary coming back and of course you've got capillaries coming back from every part of this right so all of these have been doing the same thing kind of going inside of this pool of blood and actually it 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 something like this where it really kind of has lots of surface area and it comes back around and you have the blue vessels kind of going into each one of these little finger-like projections and you have a lot of gas exchange happening so this would be maybe a better way of drawing it but the concept is the same right you basically have all this wonderful diffusion happening and as a result you actually have nice rich oxygenated blood returning to the baby so this is finally allowing the baby to have access to oxygen and nutrients and get rid of its waste its carbon dioxide and so each one of these is actually doing the same thing with red or oxygenated blood coming back and instead of having two vessels that all kind of dumps into one vessel down the middle of this vessel as we already know is called the umbilical vein so this is actually the umbilical vein now if we called the bottom one the basal plate then of course you're guessing that there must be some name for this and this of course has its name called the chorionic plate chorionic plate and this one you can already tell this has all the cells from the fetus so this belongs to the fetus so you've got some tissue that belongs to the fetus so that would be the chorionic plate in all of its contents you've got some tissue that belongs to mom that's the basal plate and then you've got some shared tissue or space in here because all of these little projections 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 so I kind of think of the placenta then is kind of the first time that mom and baby work together for a common purpose