In this video, we're going to be talking about how changes in arteriolar diameter can affect glomerular filtration rate. Now, what is actually happening is that changes in arteriolar diameter of either the afferent or efferent arteriole directly affect the glomerular filtration pressure, which then in turn directly affects the glomerular filtration rate. As remember, those variables are positively correlated. So if one goes up, the other goes up. If one goes down, the other goes down.

Now, as I've mentioned a few times, I know that the anatomy in this chapter can be a little confusing looking. I know that there's a lot of moving parts to keep track of. And when I was first learning this, it really helped me to kind of just take a step back and use a metaphor to help me understand what was going on in my head because it can be confusing. So, I want to share that metaphor with you here, and hopefully, it can be helpful to you. So bear with me for a moment, but I want you to imagine that the glomerular capsule is a sink. And in our sink metaphor, the faucet is going to be the afferent arteriole. Right? So the faucet is where the water would arrive; the afferent arteriole is where blood would arrive. The basin of the sink is going to be the glomerulus, so our big old ball of capillaries. And then the drain, where water would exit, is going to be our efferent arteriole, where blood will be exiting from.

We're going to go through what vasodilation and vasoconstriction of the afferent and efferent arterioles would look like within this metaphor and how they would be affecting the glomerular filtration pressure and glomerular filtration rate. So, if we were to have vasoconstriction of our afferent arteriole, I want you to imagine that we have basically turned the faucet down. So as you can see now, very little water is coming out of that faucet and not much water is going to accumulate in that basin. And that's what's happening in our glomerulus because of the constriction of that afferent arteriole, blood just can't really get into the glomerulus in the first place. And so, not very much is accumulating in there. And because there's just less blood, we're going to have a decrease in filtration pressure, which will lead to a decrease in filtration rate.

Now, if we were to have vasodilation of our afferent arteriole, I want you to imagine that we are now turning the faucet up. So now water is pouring into the sink, and it's pouring into the sink faster than it can drain, so the basin is beginning to fill up a little bit and that's what's happening within our glomerulus. So if our afferent arteriole is dilated, a ton of blood is rushing into that glomerulus faster than it can actually drain out. And so we're going to have an increase in pressure and an increase in filtration rate as a result of that.

Now, switching gears to our efferent arteriole. If we were to have vasoconstriction of our efferent arteriole, I want you to imagine that our drain is now clogged. So our drain is clogged, and if we were to run the faucet at all, the water would just be accumulating in the basin because there's nowhere for it to go to be draining out of. And that's what's going to happen in our anatomy. So if we were to have constriction of that efferent arteriole, blood just can't get out of the glomerulus. And so we're going to have a backup where blood is still coming in, but it can't get out. And so that's going to lead to an increase in pressure as well as an increase in filtration rate.

Now, if we were to have vasodilation of our efferent arteriole, I want you to imagine now that somebody built this sink, but they did not do a very good job. And the drain of the sink is just way too big. And so no matter how high you put that water on, water can never accumulate in the basin because it's going to just go straight out of that drain. That's kind of what's happening here. Our efferent arteriole is now so dilated that blood is just rushing out of the glomerulus. And so because the blood is rushing out and not really accumulating in there, we're going to have a decrease in glomerular filtration pressure and a decrease in glomerular filtration rate.

Hopefully, this is helpful to you. We did include some images of sinks in some upcoming videos as we go over more specific regulation mechanisms to help you track the metaphor as we go along, and I will see you guys in the next one. Bye bye.