Alright. Let's get into reabsorption in our nephron loop now. So for the most part, reabsorption in the nephron loop is going to be employing the same mechanisms that we saw in the proximal tubule. However, the permeability of our descending and ascending limbs actually differ. So, they are basically permeable to different substances. Now, one thing to note is that about 65% of the filtrate gets reabsorbed in our proximal tubule and that includes about 65% of water and 65% of ions, as well as over 99% of organic solute. I'm going to write that like this. So that includes things like glucose and amino acids. So for all intents and purposes, organic solutes are basically out of the filtrate by this point. And once filtrate reaches our nephron loop, it is mostly water and ions with sodium, of course, being the most abundant of those ions. Of course, there's also some waste products in here as well like urea, which don't get reabsorbed. So mostly water and ions. So, we're going to go through our descending and ascending limbs separately talking about what water and ion reabsorption looks like in each one.

Starting with our descending limb. Our descending limb contains abundant aquaporin. And so, in this limb, water is going to get reabsorbed via osmosis the exact same way that we saw in the proximal tubule. However, our descending limb is impermeable to ions. There are no cotransporters. There is no sodium-potassium pump. There is none of that. Thus, there is no ion reabsorption happening in the descending limb at all. So, if you look at our image here, we have the capillary over on the left. We have the lumen of the tubule here on the right, and our little tubule cells. And they do look quite different than what we just saw previously. The tubule cells in the descending limb are quite skinny and small, so that's why they look so different. And you can see we've got aquaporins all along that basolateral membrane and a ton of water movement happening. And about 20% of water is going to get reabsorbed in our descending limb. And, again, no ion reabsorption at all happening here.

Now, moving over to our ascending limb. Here, aquaporins are going to be very scarce. And so, basically, osmosis does not occur and we really don't have any water movement happening here. However, there is plenty of ion movement happening. So we're going to have some secondary active transport happening along our apical membrane and some primary active transport happening along our basolateral membrane. And so, if you look at our image, we have the lumen of the tubule and you can see it's all full of ions. We've got our purple sodium, our pink potassium, our little orange chloride there. I mean, we've got cotransporters all along that apical membrane, and we've got our sodium-potassium pumps along that basolateral membrane there. So we've got all kinds of active transports of ions happening and about 25% of ions are going to get reabsorbed in the ascending limb of the nephron loop.

Alright. So that is our reabsorption in the nephron loop. And I'll see you guys in the next video. Bye bye.