25. The Urinary System
Renal Physiology Step 2: Tubular Reabsorption
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Okay. So let's get into the passive reabsorption of water in our proximal tubule. And we're also gonna be talking a bit about solutes here as well, because what happens in the proximal tubule is that water and solute reabsorption follows kind of a cyclical pattern. So first, like we just talked about, we're gonna see all this movement of sodium and other solutes. You can see here in our capillary, we have all of this sodium and glucose, and all of these solute have left the tubule cell, and they're now hanging out in the interstitial distal base. They're gonna hang out in that capillary. And the movement of all of those solutes is going to establish a strong osmotic gradient. Remember, water wants to move toward high concentrations of solute. So, now all of our little water molecules over here are being drawn toward those high solute concentrations in the interstitial space and the capillary. And so, this osmotic gradient causes water molecules to be reabsorbed via aquaporins. You can see we have our little aquaporins here on our apical membrane of our tubule cell as well as our basal lateral membrane, and those water molecules will just travel right through those aquaporins following that osmotic gradient to get them closer to those high, solute concentration. And this is sometimes known as obligatory water reabsorption. And it's called that because the water is basically obliged to follow that osmotic gradient. So now we have a whole bunch of water hanging out in our capillary, and that is going to create a steep concentration gradient for other solutes in the filtrate because now the filtrate is becoming more and more concentrated. And those solutes want to escape the high concentrations and they wanna go toward low concentration. So these solutes essentially follow the water. And so all of these solutes over here in the filtrate are going to be drawn away from these high concentrations toward low concentrations. And so you can see how it basically, the reabsorption of solutes and water essentially creates a cyclical pattern that drives really, fast and very efficient reabsorption in the proximal tubule. And that is how we get the whole much of that filtrate reabsorption happening in that very tiny section of our nephron. Alright. So I'll see you guys in our next video. Bye bye.
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