Current timeTotal duration Google Classroom Facebook Twitter. Video transcript Think back to the kidney, and we talked about how there's an efferent and afferent arteriole. This is the afferent arteriole going towards the glomerulus, and there's a whole clump of blood vessels here. And then there's the efferent arteriole that leaves that clump of blood vessels. And those blood vessels we know are going to be surrounded by the Bowman's capsule. And we named all the different parts of the nephron, the proximal convoluted tubule, the loop of Henley, and then this is the distal convoluted tubule.
And I'm drawing it in between the afferent and efferent arteriole on purpose, and this is where all the different distal convoluted tubules meet up into that collecting duct. And in this video, I want to basically expand on this little piece, where the efferent and afferent arteriole are coming together into the glomerulus and, in between them, how there's that distal convoluted tubule. So just keep that picture in mind as I start expanding this drawing. So over here, let's start with the afferent arteriole.
I'm going to start drawing it, hopefully I'll have enough space here, something like that. And these are the endothelial cells here that are lining that blood vessel, that arteriole. And on this side, we have the same endothelial cells, of course. But now it's leaving the glomerulus. So we've got coming and going. And over here, this is the efferent arteriole.
And, of course, the other one would be the afferent arteriole. And in fact, I'm going to reverse this arrow just so there's no confusion about direction of blood flow.
I don't want you to be confused about where the blood is flowing. It's going to be going like that, and this is the afferent arteriole. So I've got my blood vessels labeled. And between the two, I also have the distal convoluted tubule, so let's draw that in. And this is the cells surrounding that distal convoluted tubule. There it is. And there's some very special cells also in here, and I'm going to draw in a different color.
And they are the macula densa cells. It's actually part of the tubule, but they're very special. So I'm going to draw them for that reason. So this is the distal convoluted tubule. And in green, I said the macula densa cells. A lot of names I'm throwing at you. And I want you to start feeling comfortable with these names, because they're actually going to be used quite a bit. Macula densa cells. 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 up at you.
Now, the next thing I want you to think back about and remember is that arterioles don't have just one layer. I mean we know that arterioles have multiple layers. The inner layer, the tunica intima, is the endothelial cells, we know that. But there's also smooth muscle cells.
We know that there's also a layer called the tunica media that's in here with smooth muscle cells, and I'm going to try to draw some smooth muscle cells right there. So we have a layer of these smooth muscle cells. And if you look closely under a microscope, you'll see that there's also some interesting cells right here.
And I'm drawing them in blue just to highlight that they're different, but they are actually very similar to smooth muscle cells. And so in a way, they're specialized smooth muscle cells.
So let me label these two new cell types I've drawn for you. Actually, I'll label them down here. Smooth muscle cells. And 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 muscles cells, and then you have these juxtaglomerular cells. Talk about a funny word, huh, juxtaglomerular cells. So juxtaglomerular cells are there. And if you looked under a microscope, they'd be full of granules.
And so sometimes actually they're even called granular cells. And so let me draw in some granules just to remind you that that's what people see under a microscope, little green granules in this case.
And I'll put them into all of them. And you know that these cells are on both sides of the vessel because, of course, we cut it long ways. So we're just looking at it as if it's disconnected. But you know these two sides are obviously touching if you thought of it in three dimensions.
And now I've talked about four cell types. Let's round it out with the last cell type. This is in orange now. This is the mesangial cell, and mesangial cells are really 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 and structurally sound, so think of them as being there for support reasons.
So these are the mesangial cells. And so combined, if you think about all this stuff together-- remember, this is all the white box in the little picture kind of blown up. If you think about all this stuff together, the macula densa cells-- we've got the endothelial cells, the smooth muscle cells, the juxtaglomerular cells, and the mesangial cells.
Put together, this whole thing is the juxtaglomerular apparatus. Kind of a funny word, but it's how people refer to all these cells. So juxtaglomerular apparatus. And the key here is remembering that the goal of the juxtaglomerular apparatus is to release renin, and so think about where renin is. Now, I mentioned these granules right here. And these are actually each going to be loaded with renin.
So these little granules, when they dump themselves into a blood vessel, this is your renin. And that renin is going to make its way into the afferent arteriole, just the way I drew it.
And then it's going to go through the glomerulus. And on the other side, it's going to sprinkle out and go out the efferent arteriole. So that's the way renin gets released. But what we haven't figured out yet, what I haven't said, is how. Why does the juxtaglomerular cells, why would it release or how does it release the renin?
Specialized cells macula densa of distal tubules lie adjacent to the JG cells of the afferent arteriole. The macula densa senses the concentration of sodium and chloride ions in the tubular fluid. When NaCl is elevated in the tubular fluid, renin release is inhibited. In contrast, a reduction in tubular NaCl stimulates renin release by the JG cells. When afferent arteriole pressure is reduced, glomerular filtration decreases, and this reduces NaCl in the distal tubule.
This serves as an important mechanism contributing to the release of renin when there is afferent arteriole hypotension, which can be caused by systemic hypotension or narrowing stenosis of the renal artery that supplies blood flow to the kidney.
When renin is released into the blood, it acts upon a circulating substrate, angiotensinogen , that undergoes proteolytic cleavage to form the decapeptide angiotensin I. Vascular endothelium, particularly in the lungs, has an enzyme, angiotensin converting enzyme ACE , that cleaves off two amino acids to form the octapeptide, angiotensin II AII , although many other tissues in the body heart, brain, vascular also can form AII.
The renin-angiotensin-aldosterone pathway is not only regulated by the mechanisms that stimulate renin release, but it is also modulated by natriuretic peptides released by the heart.
These natriuretic peptides acts as an important counter-regulatory system. Therapeutic manipulation of this pathway is very important in treating hypertension and heart failure. ACE inhibitors , AII receptor blockers and aldosterone receptor blockers , for example, are used to decrease arterial pressure, ventricular afterload, blood volume and hence ventricular preload, as well as inhibit and reverse cardiac and vascular hypertrophy.
Cardiovascular Physiology Concepts Richard E.
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