Changing glomerular filtration rate
About this video
This video explains how the glomerulus filters blood. Small molecules such as ions, amino acids, glucose and water leak through the fenestrations of the capillaries into Bowman's space, while the basement membrane and podocytes keep blood cells and large proteins out. The rate of filtration depends on the diameters of the afferent and efferent arterioles: a wider inflow or a narrower outflow increases filtration, and the reverse decreases it, as in renal artery stenosis.
You'll be able to
- describe which substances pass into Bowman's space and which structures keep blood cells and large proteins out.
- predict how changing the afferent or efferent arteriole diameter changes filtration rate.
- apply the afferent and efferent logic to a clinical case such as renal artery stenosis.
Key words
- Glomerulus
- A tuft of capillaries where blood is filtered into Bowman's space.
- Bowman's space
- The space around the glomerulus that collects the filtered fluid, where processing of the filtrate begins.
- Afferent arteriole
- The vessel that carries blood into the glomerulus.
- Efferent arteriole
- The vessel that carries blood away from the glomerulus.
- Renal artery stenosis
- A narrowing of the artery feeding the kidney, which reduces blood flow into the glomerulus.
Transcript
Voiceover: All right, so I think we have a pretty good appreciation of how we have some smaller ions, amino acids, glucose and even water than can leak through these fenestrations and get into this space right here. Remember, this is just Bowman's space. They can get into Bowman's space where we can then process them into the rest of the nephron.
This is Bowman's space, and we have these podocytes that hug the arterioles right here by holding on to the endothelial cells. We also have this basement membrane that prevents giant proteins in our red and white blood cells from leaking through. And we have these tubule cells that help line the other side, and they're epithelial cells, they're in the class of epithelial cells. Excellent.
Now the last thing I want to cover is what's happening here from having an arterial go into a structure and then give off another arterial. That's a little weird, right, because we usually like thinking about arterioles going into capillaries and then giving out a venule.
Well one way to think about this is sort of like a marathon where there are a lot of people running on the same street from point A to point B, and they're either running in a narrow street or a wide street. Let's make a table to think about this. What would happen if we look at our afferent and our efferent arterioles, and we change the diameter of the vessel.
What does that do to the rate of filtration? We'll write filtration right here. How does the diameter of our vessels change the rate of filtration? If we increase the diameter of the afferent arterial, or if we have a very wide path that allows a lot of marathon runners to run into the glomerulus, that means there's going to be a lot of blood here that includes all of the ions, and the amino acids and glucose, in addition to our blood cells and giant proteins we talked about.
There's going to be a lot of stuff over here. So if there's a lot of stuff running over these fenestrations you're going to have a lot of leakage, and so there's going to be a lot of filtration. More filtration occurs if there are more people or more marathon runners running into this space right here, so more filtration.
What about if we did that with the efferent arteriole? Let's say we increase the diameter of the efferent arteriole so there's space for more marathon runners to run away from this very narrow street, or this place where a lot of people can run off the pavement and get into these holes and go elsewhere.
If we increase the diameter of our efferent arterial and allow people to leave, they're not going to be able to stay around here for a long period of time, they're not going to be near these fenestrations. That means that our filtration rate will decrease, because the blood is moving away from the place that it would be filtered.
The same thing goes if we decrease the diameter of our afferent arteriole. If we decrease the amount of runners or blood that can come into the glomerulus, that means there's going to be less fluid filtered out, so a lower filtration rate. This is actually what happens with renal artery stenosis.
If we have a very narrow or stenosed vessel - that's what stenosis means, it's just narrow - renal artery, that means there's going to be less blood that branches off and goes to our afferent arteriole. There's going to be less blood that runs across our fenestrations and is filtered away.
On the flip side, if we decrease the diameter of our efferent arterial that makes it difficult for our runners to leave this fenestrated vessel, so there's going to be some backup, there's going to be a lot of ions and amino acids and glucose hanging around here near these holes.
If blood backs up that means so do these guys, so then they're going to be filtered through these holes and collected into Bowman's space, so our filtration rate will increase, it will increase because there's a lot of backup that allows more time for filtration.
Maybe if you're interested in learning about kidney diseases later on there's a lot of stuff that tweaks this system, that messes around with the afferent arterial or the efferent arterial, sort of like what I talked about for renal artery stenosis. I encourage you to think about what can go wrong with this process and how things can change, that's kind of the best way to learn about it.
That's how our glomerulus works. Let's move on to the next part of our nephron.
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