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Glomerular filtration in the nephron

Created by Raja Narayan.

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Video transcript

Voiceover: All right, so I think we have a pretty decent appreciation of renal anatomy; we know how the kidney is structured, now we just need to take a look at some of the finer details. We started talking about the nephron, which I kinda drew right here, and we said, "This is the functional unit of filtration "and collection in the kidney." So let's start off with the very beginning of the nephron. The first part of the nephron is called the "glomerulus"; it receives branches that come off the renal artery, you see a branch going that a way; there's a branch going this a way, and just like any artery, it branches off into arterioles, and it's an arteriole that comes up first to meet the glomerulus. So if we look down here, we're going to have something that came off of the renal artery, that's an arteriole, so I'll write, "arteriole," right here, and we actually further specify this: we call this the "afferent arteriole," "afferent" meaning, "going towards." And so, this is the afferent arteriole, or the arteriole that's going towards the glomerulus. The glomerulus then, is this really loopy structure; there's a lot of spinning that goes on here, then we branch off again, and this gives us the same arteriole, this is the same vessel we just started off with, so we're going this way, and spinning around and coming out, as one single vessel, but we call this part of it, the "efferent" arteriole: "efferent," meaning that we have left the glomerulus. And that of course leaves this ball-like structure over here, that's going to be known as the glomerulus. Now the thing about the glomerulus that's really interesting: it's the main site for filtration, where we take blood that came in from the renal artery, and we push out a whole bunch of fluid, that we're then going to take out some ions, and some water, and some waste, and we'll get rid of the waste or the extra ions. The glomerulus is where we take blood and turn it into filtrate, and let the rest of the blood flow on. So this efferent arteriole is gonna turn into a capillary, and then it's gonna go into venules, and then collect back, and come out as the renal vein; we'll talk about that in a later video, when I talk about other parts of the nephron. The glomerulus though, just leaks out fluid, and it needs to be caught somewhere. That fluid that leaks out is caught in a capsule, that's kind of hugging the glomerulus right here. So I'm gonna draw it, like that, and it kinda keeps going this way, and this is gonna continue on, into the rest of our nephron, but this thing right here, it's a capsule, and actually it has a name; it's named after a British scientist, "Doctor Bowman," so we call this, "Bowman's Capsule." This is Bowman's Capsule, and this is where we're going to collect the filtrate, or the fluid that comes out of the glomerululs. The inside right here is just open space, so they call it, "Bowman's Space" as well, so it's just space that's gonna collect our filtrate. So at this point, you should be asking yourself, "Why is it that we're gonna have fluid leak out here? "I mean, there's all this wrapping that goes around, "so we've got high pressure, but how is this different "from other arterioles in our body? "Why is it that we have so much leakage, "purposefully happening here, "but it doesn't happen everywhere else in our body?" So let me answer your question, and why don't we just blow up that part, right here, and open this window so we can take a better look. So the point where the arteriole meets Bowman's Capsule, there's a lot going on. Recall, that when we have a vessel, I'll draw half of it, like that, right there, and it's kind of going this a way, okay, so that's our vessel that's right here. This vessel's got a lot of good stuff, like our red blood cells, our white blood cells, platelets, some really really big proteins, so I'm just gonna draw something really big, right here; that's a giant protein, and it's not gonna leak out into our Bowmans' Capsule. So, this stuff kinda moves along that way, then again, we've got other things like ions, so I'm gonna write, "Sodium" right there. We've also got smaller protein sub-units, like amino acids; I'll just write, "AA," and we've also got glucose in here. These are things that can leak out, so how is it they get from the arteriole, into Bowman's Space? So our vessels, our arterioles, just like anything else in our body; they're made up of cells. And the cells that line our vessels over here, I'll just draw a whole bunch of these guys, kinda hanging out, so these guys are called, "endothelial cells"; each of these is an endothelial cell. So an endothelial cell is a lot like most of our eukaryotic cells: They've got a nucleus, and they've got all their organelles, and stuff like that goin' on; I'm not gonna go into that kinda detail for right now, but just recall that they're eukaryotic cells. Now something that's special about these vessels, is that they're fenestrated. Write that in parenthesis, "fenestrated," and if you don't know what this term means, all it means is that these vessels have a lot of holes; they're very "holey," and so, because of that, the holes allow small things like sodium, and amino acids, and glucose to leak through, so it's got some holes in them, you know, the way that they're sort of connected. So there are holes where these guys can kinda slip through. And actually some of these holes can allow bigger proteins to come through, but these proteins still don't, because there's another added layer, that sits in between these endothelial cells in the in the tuble, so this is sort of another membrane that's right here. I'll just kinda draw it shaded in, like this, and it's not a complete barrier; it's semi-permeable, meaning some things can leak through, but this is another membrane, that we call, the "Basement Membrane"; this is a basement membrane, and you may have heard about this in other contexts. So the basement membrane right here, helps to make sure that small things pass through: things like sodium can get through these fenestrations, and leak out; our amino acids can do the same; and our glucose can, at times, as well; but these bigger proteins bounce back; they bounce back, because either they can't make it through the fenestrations, or the basement membrane prevents them from leaking into Bowman's Space. And then finally, we've got the tubular cells, tubular cells that make up the interaction point on the end of the Bowman's Capsule. So they sort of look like this; they're pretty long cells, and the funny thing about them is that some of these guys actually hug the vessel; they hug the endothelial cells, like that. And so, they're sort of like these legs; this is sort of a leg-like projection, and so, if you remember a doctor you might see, if you've got problems with your feet is a "podiatrist," and so this type of cell, we call these "podocytes" right, "podo" meaning "foot." Podocytes, and so there are some podocytes, in addition to these tubular cells, there are some that are just tubule cells. And another term for that, is just an "epithelial cell"; this is an epithelial cell, okay? And so, we go from the endothelial cell, to the epithelial cell, and I think I should also mention that these podocytes are a certain class of epithelial cell, as well, and so, these guys hug around the arteriole; that sort of helps for this connection to stay close.