 It's called filtration. You are going to take fluid out of your blood and filter it into the nephron tubule. And are you ready for the craziest fact ever? You have three liters of blood in your body. Isn't that crazy? Just kidding. That's not the craziest fact ever. In 24 hours, you filter 180 liters of fluid out of your blood and into Bowman's capsule every 24 hours. What? You have got to be kidding me. That means that you filter out 60 times the entire blood volume in one 24-hour period. That hopefully makes you go, that certainly is not sustainable, and we better reabsorb most of that, which is why you have a nephron and which is why we have another entire lecture on kidney function. But filtration, you should be thinking right now, dude, something is seriously wrong. You can filter out 180 liters of fluid. Well, there's a couple of factors that come into play. First of all, as you know now, the structure of the renal corpuscle facilitates filtration. Leaky capillaries, leaky visceral layer of Bowman's capsule. It's a nice colander-type situation for filtration to take place. But we're actually going to look at the pressure dynamics involved in this, because we actually have to have some weird pressures going on in order to make this work. So I am drawing my whole glomerulus, remember this little phenomenon and this giant, okay, it got a little narrow down here, but just pretend like it isn't. Pretend like that's like shading and perspective. Totally did that on purpose. Remember that this is my afferent arterial and this is my efferent arterial. And I'm also going to draw, this is my glomerular capillary bed madness. I'm going to draw you, for perspective, a regular capillary. We've talked about this already. We've talked about how a capillary exchange happens because of different pressure dynamics in the capillaries. And I want to compare the regular capillaries to the glomerular capillaries from a pressure perspective. Do you remember that we actually took hydrostatic pressure? I'm going to give you some numbers for hydrostatic pressure in a regular capillary. Hydrostatic pressure, remember, was just blood pressure from the heart, the beating of the heart. And the beating of the heart sends blood through the kidneys. So we're going to have hydrostatic pressure in our glomerulus. That pressure pushes fluid out. And I don't think I gave you numbers in the last, in that lecture, but essentially it's 32 millimeters of mercury in a normal capillary. And we're going to experience the same thing. We're going to have about 32 millimeters of mercury of pressure from the beating heart, pushing fluid out into the Bowman's capsule. Something that you, we talked about, that our hydrostatic pressure over time due to friction decreases. It actually goes down to about 15 millimeters of mercury over time. We don't see a change here. I mean, the glomerulus is a blob. It's like a knot, a single knot. We don't have a length of effect. So we're not going to see a decrease like this. But why don't we filter out 180 liters of fluid into our regular little bloodstream, into our regular capillaries and our tissues? It's because, remember, we had another kind of pressure drawing stuff in. This pressure is steady. This was osmotic pressure due to plasma proteins. We'll do it due to PPs, the plasma proteins. And this is about 30, 25 to 30 millimeters of mercury. And if you remember, as an end result in our normal capillaries, we have net filtration of about three liters per day. So really that's your entire blood volume that is filtered out because of the interaction between the hydrostatic pressure and the osmotic pressure pulling the fluid back in. We end up with what happened to the three liters. It goes into the lymphatic system and then ultimately gets dumped right back into the circulatory system. And here we also have hydrostatic pressure. And it's the same, I mean, not hydrostatic, but osmotic pressure. We're going to pull our fluid back in and it's going to be about 30 millimeters of mercury pulling our fluid back in. There's actually a little bit of hydrostatic pressure that pulls fluid or that pushes fluid back in. And I'm using the same color. It's about 15 millimeters of mercury. And I want you to think about this for a second. That's actually due to the fact that there is fluid here in Bowman's capsule. So that fluid, it's like blowing up a balloon, pushing more air into your balloon. It gets harder to blow it up as it gets fuller. There's fluid already in here. So there's going to be a little bit of back push because of that fluid. But I still don't see something that let me do some serious pumping out. We've got to have something else happen here. And you know what it is. There is an increased pressure of 23 millimeters of mercury due to the different diameters of the afferent and efferent arterioles. Now think about this for a second. With normal blood pressure, the entry door into the glomerulus is giant. It's like a garage door in a house. If there were two entries into a house, a garage door and a doggie door. Garage door, doggie door. Do you see my visual of the size difference? If I filled, I started pushing people through my garage door, like hundreds of people and pushing through the garage door. But the only way for the, and I'm just pushing them in because, dude, I can fit hundreds of people through a garage door. And the only way out is through the doggie door. What's going to happen in here? Holy pressure increase. There is going to be a significant increase in pressure of people as they're like, please let me out this freaking doggie door because I'm getting squished in with all the people who are coming through the garage door. How's that analogy for you? Bottom line. We end up with about 10 millimeters of mercury of pressure pushing out. That and the leakiness of the capillaries leads to 180 liters of filtration. Holy madness. This sets you up to appreciate and understand we better reabsorb that. And we're going to spend the next entire lecture talking about how we reabsorb it. There is a concept called glomerular filtration rate, GFR, and we're going to talk about that next. How quickly, at what rate are we filtering? Because think about the kinds of things that could change these dynamics. If you increase blood pressure, if you decrease plasma protein concentration, if you decrease plasma protein concentration, you're going to decrease this number right here which is going to increase the filtration rate. If you visualize that, there are things that will change glomerular filtration rate. If you increase the filtration rate, you're going to end up filtering out more than 180 liters. If you decrease, you're going to end up filtering out less than 180 liters. And there are some homeostatic controls to kind of regulate that. So we're going to talk about those next.