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Video transcript
So here we are. We're looking at the placenta. I'm just going to outline the parts that are specifically considered the placenta. That's this bit right here. And I'm going to orient you by labeling the pink wall that's around this. You remember that's called the uterus wall. That's this wall right here. This is our uterus, and this is a very muscular organ. Remember, this is going to help us squeeze the baby out. Or it's going to help mom squeeze the baby out, anyway, when it's time for delivery. Now, in the placenta, there are different parts. And there's an inside part. Remember, mom is on this side, and baby is on-- or fetus is on this side. And when I say baby and fetus, I kind of mean them interchangeably. I recognize that they're not exactly the same thing, but when I mean baby, for the purposes of this video, I mean the unborn baby, the baby that's still getting nutrition from mom. So this layer then, this little layer that I just put in here, this is the basal plate, remember. So this is mom's tissue. And on the other side, we have the chorionic plate. Remember, that's the fetus tissue. The chorionic plate is full of lots of blood vessels. And in between the two-- this is the interesting part, where all the magic is happening. In between the two, remember, there's blood. There's a giant pool of blood, and it's full of oxygen and nutrients, and it takes away waste, like carbon dioxide. All the things that the baby needs are located in this pool of blood. And you remember, the pool of blood, the way we even got this pool of blood is that we have little arteries here, uterine arteries that are pushing blood into that pool. And then it's getting recycled by all these uterine veins. So you've got this circulation happening on the mom's side. And within the pool of blood, remember, the fetus is actually sticking its little capillaries in there. So the fetus is sticking its capillaries into the pool of blood. So that middle area, this middle zone, is really where the diffusion is happening. Remember, the diffusion is happening from the pool into the fetus's capillaries, capillaries right here. So fetus capillaries are pushing their way into that pool. And that's where they're getting their oxygen nutrients and dumping off the carbon dioxide. So the question is, how does it go from there back to the fetus? So the fetus, of course, has umbilical arteries and veins. And this red one is the umbilical vein. And how does the oxygen get from where it is now all the way back to the different organs? And I'm going to draw this little yellow circle. You can imagine this is, let's say, the belly button. And it's interesting because so much used to happen in the belly button for the fetus, and by comparison, not much happens in adult humans with the belly button at all. So you've got, let's say, one artery there, one artery there. And I'm trying to draw a little face here for us to remember what this looks like. So this is our little face with two eyes. So the umbilical vein is in red and that's-- just to label it-- this guy. And then you've got the two umbilical arteries, and those are in purple. And those are the two eyes of our umbilical cord, umbilical arteries. So what I want to do now is to figure out exactly how does blood, as I said, go from the umbilical vein all the way back to the rest of the body. And to do that, what we need to do is actually sketch out what's going on in the body. So I'm going to draw the diaphragm. Remember, the diaphragm is a muscular-- a big muscle, rather. And It's very muscular, of course, lot of skeletal muscle in here. And above the diaphragm and below the diaphragm are a couple of organs that we're going to have to talk about. And the first one, above the diaphragm, is a four-chambered organ-- I'm sure you're very familiar with this guy-- called the heart. So the heart is sitting up here, and it's got four chambers. And I'm going to quickly label the four chambers. You've got the right atrium, and the right ventricle down here, the left atrium, and the left ventricle down here. And so this is my heart. And the other organ-- and actually, let me label this muscle just in case we lose track later. This is our diaphragm, of course. And the other organ-- not muscle. The other organ is the liver. This liver is this huge organ, actually sits right below the diaphragm. So it's actually quite close to the heart, as it turns out. And the liver does a number of important things. But right now we're just going to talk about the anatomy of what's going through the liver. So you've got a vein here. I'm going to label it here. It's called the portal vein. And don't worry so much about the names of things, but I just want to diagram out how blood is traveling. And you've got another vein here I'll draw a little bit longer, just to give me more space to write. And this is the inferior vena cava. You remember this vessel. This is the big vein that brings in a lot of blood from different parts of the body. And you have the aorta. And remember, the aorta is actually going to go off the left ventricle, and it's going to go behind the heart. So I'm going to just draw dashed lines. And let's say it comes through here. And I'm showing it-- go down, down, down, down, down, and split up. It's going to split up at the legs. So this is where the legs are. And actually these internal branches, let's say this branch right here and this branch right here, these branches are called the internal iliac branches, or internal iliac arteries. You call them internal iliac arteries. And I'll explain in just a moment why I bothered to name these guys as opposed to all the other arteries and branches I could have named, because, of course, there are many, many branches off the aorta. But these are going to be very important for how the fetus's blood gets around. So let's follow blood now. Let's start back at the umbilical vein and see what happens. So the umbilical vein blood I'll just show going this way, and this umbilical vein blood is now on the inside of the body of the baby. So if I said this is the belly button, then, of course, that must mean that everything on this side of the belly button is the body of the baby. So we've got the baby, there, and it's going inside the baby's body, and the vein travels, travels, travels. And it has a choice. It can either go over to the portal vein, or it can go over to the inferior vena cava. And we'll follow one path at a time. Let's do the inferior vena cava path first. When the oxygenated red blood comes and mixes with the deoxygenated blue blood, it actually forms kind of purplish blood. That's why I'm drawing it this way. Remember, purplish represents somewhere in between having a lot of oxygen and having a very low amount of oxygen, somewhere in between. So that's why I drew that red and blue turning into purple. So if I have purplish blood there, that is one of my two paths I said. In fact, this part of the journey, this part, is only possible because there's a tube there. And that tube is actually called the ductus venosus. And essentially what this is is a shortcut. Now, if that's the shortcut, what's the other path? Makes you wonder, right? Well, the other path is actually much, much longer, obviously. And I'm going to show it over here. So if it joins the portal vein, instead of going down the ductus venosus, if blood goes this way, then it's going to branch out into different parts of the liver. It's going to go to all the different parts of the liver. And then it's going to go into capillary beds. So it's going to go and get very, very, very tiny. So let's follow one branch, let's say this guy. It's going to get into very, very, very tiny capillary beds. And then on the other side, it's going to get picked up by tiny capillary beds, and it's going to get brought back. And it's going to join up with blood from all the different parts of the liver. And it's going to eventually be part of a vein. And this vein right here is called the hepatic vein. So either way, even if it takes a shortcut or doesn't take the shortcut, eventually blood is going to make its way back into the inferior vena cava and into the right atrium. So overall, the goal is the same. It's going to get to the right atrium. But one way, taking the ductus venosus, is much quicker than the other way. And that's important, because you want-- from the fetus's perspective, you want this wonderful umbilical vein blood, which is rich and full of oxygen, to make its way back to the heart quickly so that it can get pumped out through the aorta to the rest of the body. Now there is a very interesting way that blood makes its way through the heart itself. And we're going to skip over that for the time being. I'll get into that in another video. But it gets into the aorta. That's the next part. And it goes down the aorta. Blood gets pumped to all different parts of the body. And I'm drawing it as purple, still, because, of course, it's not very, very oxygenated like the umbilical vein is, but it's also not completely deoxygenated like in the inferior vena cava. It's somewhere in between. And blood goes down into the internal iliac arteries. And from there, you actually have branches that go to one umbilical artery on this side and one umbilical artery on the other side. So you actually have branches that go off of the internal iliac artery and go to the umbilical arteries. And, of course, from that point, you know what happens. Then both arteries go into the pool of blood that mom has set up for the fetus and exchange at the capillaries for oxygen and nutrients. And the new blood, then, is very, very highly oxygenated. And the journey repeats itself. So just to make sure we're clear, let me just start from the beginning and go through it again, Now I'm going to use a little blue dot to track-- oh, let's say a little green dot-- to track where we are. So blood starts in the umbilical vein right here. And let's say that's our first starting point. It goes to a little branch point, where it either can go down into the ductus venosus-- that would be this one. Or it can go this way, and join the portal vein. It has an option. If it goes the long way, then it goes into the portal vein, and it goes off into some capillary, gets picked up and dragged back, and joins the hepatic vein, which eventually also merges with the inferior vena cava. Of course, if it takes the shortcut, that route is much quicker. So that's the whole point of the ductus venosus, is to skip over the liver. Eventually the blood makes its way to the right atrium. And again, it's going to have an interesting path through the heart, which we'll get into in a future video. But for right now, let's just assume it gets through the heart and gets out into the aorta. Now blood is going to go down the aorta, is going to branch off into one of the two legs, because these branches go to the two different legs of our fetus. And it'll go down into the internal iliac artery right here and on the other side right here. And at that point, it'll shoot up-- some of it, anyway, will shoot up into this umbilical artery, into these umbilical arteries, these two. So now blood is over here into our two umbilical arteries. And it's going to go down, and it's going to exchange inside of that pool of blood in the placenta, and it's going to turn around and join our umbilical vein again. So that's the loop that we follow, and that is the fetal circulatory system.