We talked about how there's an efferent and an afferent arteriole. This is the afferent arteriole, going towards the glomerulus,
there's a whole clump of blood vessels here. There's the efferent arteriole which
leaves that clump of blood vessels. Those blood vessels we know are gonna be
surrounded by Bowman's capsule. We named all the different parts of the nephron. The proximal convoluted tubule,
the loop of Henle, and this is the distal convoluted tubule. I'm drawing it in between the afferent and efferent arteriole on purpose. This is where all the different distal convoluted tubules
meet up into that collecting duct. In this section, in this little video,
I want to expand on this little piece. Where the efferent and afferent arteriole
are coming together into that glomerulus between that there's that distal convoluted tubule. Just keep that picture in mind
as I start expanding this drawing. Over here we have the afferent arteriole,
I'm gonna start drawing it, I have enough space here. Something like that. These are the endothelial cells
that are lining that blood vessel, that arteriole. On this side we have the same endothelial cells of course, but now it's leaving the glomerulus. We've got coming and going,
over here is the efferent arteriole. The other one is of course the afferent arteriole. I'm gonna reverse that arrow, just so there's no confusion about the direction of the blood flow. I don't want you to be confused
about where the blood is going. It's gonna be going like that
and this is the afferent arteriole. I've got my blood vessels labeled. Between the two I also have the distal convoluted tubule,
so let's draw that in. This are the cells surrounding the distal convoluted tubule. There are some very special cells in here,
I'm gonna draw them in a different color. They are the macula densa cells. They're part of the tubule, but they're very special,
so I'm gonna draw them for that reason. This is the distal convoluted tubule. And in green are the macula densa cells. I'm throwing a lot of names at you
and I want you to start feeling comfortable with these names, because they're gonna be used quite a bit. It's not particularly hard once you get used to the language, but I know it can be confusing to see all these funny words thrown at you. The next thing I want you to think back about and remember is that arterioles don't just have one layer. We know that arterioles have multiple layers. The inner layer, the tunica intima,
is the endothelial cells, we know that. There's also smooth muscle cells.
We know that there's also a layer called the tunica media. That's here, with smooth muscle cells. I'm gonna try to draw some smooth muscle cells right there. We have a layer of these smooth muscle cells. If you look closely under a microscope, you'll see
that there are also some interesting cells right here. I'm drawing them in blue,
just to highlight that they're different. They are actually very similar to smooth muscle cells. In a way they're specialized smooth muscle cells. Let me label these two cell types that I've drawn for you. Smooth muscle cells, they're on the afferent arteriole side. You'll see them a little bit on the efferent arteriole side
as well, but mostly on the afferent arteriole side. Smooth muscle cells.
Then you have these juxtaglomerular cells. Juxta, talk about a funny word... Juxtaglomerular cells. All right, so the juxtaglomerular cells are there. If you looked under a microscope, they'd be full of granules. Sometimes they're even called granular cells. Let me draw in some granules
just to remind you that's what people see under a microscope. Little green granules in this case. I'll put them into all of them. You know that these cells are on both sides of the vessel,
because of course we cut it longways, so we're just looking as if it's disconnected. But these two sides are obviously touching,
if you thought of it in three dimensions. Now I've talked about four cell types,
let's round it up with a last cell type. This is in orange now, these are the mesangial cells. Mesangial cells are there for structure. They're really there to hold the whole thing together
so that the blood vessels and the nephron are in close contact. So that they're structurally sound.
Think of them as being there for support reasons. These are the mesangial cells. Combined, if you think about all this stuff together, remember
this is all the white box in the little picture blown up. If you think about all this stuff together, the macula densa cells, the endothelial cells, the smooth muscle cells, the juxtaglomerular cells and the mesangial cells,
put together, this whole thing is the juxtaglomerular complex. Or apparatus rather, the juxtaglomerular apparatus. Kind of a funny word, but it's how people refer
to all these cells, the juxtaglomerular apparatus. The key here is to remember that the goal of the juxtaglomerular apparatus is to release renin. Think about where renin is.
I mentioned these little granules here. These are actually each gonna be loaded with renin. These little granules dump themselves into the vessels,
this is your renin. That renin is gonna make its way into the afferent arteriole, just like I drew it, then it's gonna make its way through the glomerulus. On the other and it's gonna sprinkle out
and go into the efferent arteriole. That's the way renin gets released. What we hadn't figured out yet, what we hadn't said,
is how, why would the juxtaglomerular apparatus release renin. What is the trigger? Let's figure out what are the key triggers
for the release of renin. What are the triggers? There are three actually.
Three common ones we know. One is simply low blood pressure.
This cells are gonna feel mechanically less blood pressure. They're gonna say 'What's going on here?
Pressure is low, we've got to do something about it.' 'We're gonna release renin.' So one trigger would be low blood pressure. That's the first one and it's actually directly sensed
by the juxtaglomerular cells. That's actually sensed right here.
I'm gonna draw a 1 for that. The second trigger is a nerve cell trigger. Actually I haven't even drawn that in for you yet. Remember that this is a blood vessel, right here, with our two layers,
our endothelial layer and our tunica media layer. There's also an external layer, the tunica externa. We also have a blood vessel here, these mesangial cells, they're also specialized smooth muscle cells. We have these layers of blood vessels and the two blood vessels are kind of merging and fusing right here. They're coming together right here. In this external layers you actually have, I'm gonna
draw it in yellow, you have these little nerve endings. Remember that nerves can end in that layer,
the external layer. You have the sympathetic nerve endings. They come and sit with their nerve endings
right on the juxtaglomerular cells. They're sitting right there and when they fire,
that's gonna make the JG cells want to dump their renin. So the second trigger is sympathetic nerves. Now there's one more trigger, the third trigger. This one is actually a little distance away. It's the macula densa cells. I mentioned them earlier and I said that they're special. I haven't really gotten into why they're special.
Let me tell you what happens. What happens is these macula densa cells,
they're sitting there in the distal convoluted tubule, sampling what comes through.
They're just kind of feeling what comes through. They're seeing sodium come through. They're checking and checking,
is there a lot of sodium coming through, or a little bit of sodium. When they start sensing that the sodium content,
that the amount of sodium coming through that distal convoluted tubule
is really quite low, when they start feeling like
not too much sodium is coming through, they start thinking to themselves
'Why is this happening?' If you think about it, you can figure it out too. If there is not a lot of sodium here,
that's probably because there's not a lot of sodium here. And that could be because there's not
enough sodium here or here. So really, when the distal convoluted tubule
senses low sodium, it's probably related to the fact that not enough sodium is getting in at the getgo, at the point of filtration. And that could be a reflection of low blood pressure. So when the macula densa cells sense low sodium levels,
what they're sensing is low pressure in the glomerulus. So if there's low pressure in the glomerulus,
our glomerulus right here, they think
'Ok,that's probably the reason our sodium levels are low.' 'Let's send a signal out to the juxtaglomerular cells.' Low sodium in the macula densa,
picked up by the macula densa rather. It means that the filtration pressure
in the glomerulus was too low. So what they decide to do,
let me find a new color, maybe something like this. They send a little messenger. I'll do my messenger in orange. They send a messenger to go
over to the juxtaglomerular cells. That messenger is a little molecule called prostaglandin. It's a local messenger. It really doesn't act far away from these cells,
it just acts locally. This local hormone, it's sometimes called a paracrine hormone, I'll write that here, paracrine hormone, is going to send a signal from the macula densa cells
over to the juxtaglomerular cells. They say 'Hey, there's a trigger, we sensed low sodium
and we think it's because of low blood pressure.' 'Why don't you go and do something about it?' So those are the three triggers. Actually I don't think I've labeled them well. This is trigger number two and this is trigger number three. These are the three triggers that will make
the renin get released into our blood stream. This is a picture of renin, this is a picture of the molecule. I've actually already drawn kind of like
a pacman-like shape around it. When I talk about renin coming out of the juxtaglomerular cells, you get a sense of what it actually looks like. This is a three dimensional figure of this protein. Keep in mind this is a protein hormone. Meaning it's a protein that has the ability of
letting one cell talk to other cells at a distance. This is what that renin looks like. We'll discuss more about how renin works in the next video.