 Glomerular filtration rate, we've already identified. It happens at essentially 180 liters per day. That's the rate at which filtration occurs in the glomerulus, between the glomerulus and Bowman's capsule. That rate, I have to draw you this graph because this is a crazy talk. You compare glomerular filtration rate, and let's throw in, you know, glomerular filtration rate of 180 liters. You can have a mean arterial pressure between 80. Now remember, that's just the difference between, actually that was that whole crazy number that we calculated, but it's an estimate of blood pressure. Your mean arterial pressure can be anywhere between 80 and 180. That's a huge difference. And your body, your kidneys will regulate this and maintain GFR at 180 liters per day, even if your blood pressure changes. Now, there's two primary mechanisms for this, and we're going to talk about these. If your mean arterial pressure drops below 80, then you're going to see a decrease. That's going to decrease hydrostatic pressure, and you'll actually see a decrease in GFR because of that. If you go above 180, dude, that's gigantic, you're going to see an increase in glomerular filtration rate. But other than that, holy range of blood pressures and no change in GFR, how? Well, there's one mechanism. It's a myogenic reflex. Myo should make you think, what? Muscle. It's a reflex that's, there's no integrator in this reflex. It's facilitated by smooth muscle in the afferent arterial. So here's what happens. Here's my afferent arterial heading into glomerulus. Arterials are surrounded by circular layers of smooth muscle. You already know that from our study of vessel anatomy. So the smooth muscle, in addition to layers of smooth muscle surrounding my arterial, we have special sensory receptors. So I'm just going to make a little sensory receptor so that you can see that. So these are sensory stretch receptors. So stimulus, blood pressure increases. Blood pressure increases. Pressure from the blood in the vessel increases. What's that going to do to the vessel? If you have increased pressure, that's going to stretch the blood vessel. That's going to activate these sensory receptors. When they stretch, we have a mechanism. Mechanical gated calcium channels open. And this actually initiates, number one, an action potential, number two, contraction of smooth muscle. Smooth muscle contracts. And what's the outcome? Vasoconstriction. Now think about this for a second. Afferent arterial vasoconstricts. The diameter of the afferent arterial decreases. That means I just made my garage door go from a garage door to being like just a normal little joe get in your house door. And that's going to decrease the number of people that can come into my house. And then there's going to be fewer people to apply pressure to get out the doggy door. GFR will decrease. The myogenic reflex happens as a response to an increased GFR. And then the result is decreased. And think about how there's no sending the information to an integrator. There's no time, like, brain doesn't have to, I don't have to think about it. Oh, gee, I'm feeling like my blood pressure is a little... This is fast. This is instantaneous. I stretch my, in one afferent arterial, I stretch. And then I'm going to constrict just to manage that. I'm not going to let it change. That's a huge amount of power. That will keep me at a nice, steady GFR no matter what the blood pressure is around me. I can adjust. That's one mechanism. That mechanism is pretty straightforward. The next one is a little more complicated. Let's talk about what happens at the juxtaglomerular apparatus.