 Descending loop of Henlea. Proximal convoluted tubule, cells are thick, cells have a high surface area, there's lots of transporters in there, probably lots of mitochondria to power the transport, lots of reabsorption of everything happening. Descending loop of Henlea cells, they're almost like boring. Is that possible? I feel bad for saying that. No cells are boring cells. But really, it's not going to be a very pretty picture. Descending loop of Henlea is they are permeable to water, but not solutes. Okay, what's that mean? Water can pass through, but we don't have any transporters to deal with any solutes. The walls of the descending loop of Henlea are thinner and only water can come through. Now you tell me, are we going to want water to come out or are we going to want water to go into the tubule? We just filtered out 180 liters minus the 70% that we just reabsorbed. We do not want water to go into our tubule. We want to pull water out. We're going to give you a preview of something that's absolutely fundamental to understanding urine creation. And we'll talk about it at the very end of this lecture. But here it is. There is a medullary concentration gradient. So they're in this interstitial fluid here that everything is passing through. I'm going to draw some lines on this. Why? Because there is a concentration gradient. And whoa, go back to where you were. Watch this amazingness. I'm just going to tell you what is a concentration gradient to change in concentration over space. Your blood concentration was about 300 milliosmoles or .3 osmoles. That's just a concentration. Bring it back. Take a deep breath. You love concentrations. We're good. That's what we would expect. Dealing with milliosmoles makes it a little bit easier to comprehend versus .3 osmoles. So we're just going to throw all these into milliosmoles. Here's the scoop. The interstitial fluid surrounding the renal corpuscle is isosmotic to the blood. So of course it is. I mean, who would think that it wouldn't be? Well, when I say there's a concentration gradient, at the bottom of the loop of Henley, at the place where the descending loop of Henley and the hairpin turn and heads back up via the ascending loop of Henley, down at the very bottom, the interstitial osmolarity is 1200 milliosmoles. What? Seriously? I hope that that makes you go, dude, that's crazy talk. So I'm going to tell you how we got that. At this exact moment, accept it. It's there. So if the descending loop of Henley is permeable to water, but not to anything else, what's going to happen? Water is going to come out. Now in the proximal convoluted tubule, the filtrate ends up being this isosmotic to the interstitial fluid, because if it wasn't, water would leave. Or solutes would leave, which would make water leave. So because solutes and water can come out at the proximal convoluted tubule, the filtrate is isosmotic at that point. So when the filtrate enters the descending loop of Henley, it's isosmotic to 300 milliosmoles. However, look what happens when now, let's say this is 600 milliosmoles. Dude, water, it's way more concentrated outside. So water is going to leave. What's that going to do to the concentration? It's totally going to become more concentrated inside because the water left. This is perfectly clear. So more water is going to leave, because let's say this is like 800 and 100 or 1,000. And water is going to continue to leave until ultimately our filtrate is concentrated up to 1200 milliosmoles. So it's isosmotic with the interstitial concentration gradient. Now, think about if we didn't have that, could you reabsorb all that extra water? No, but in the descending loop of Henley, we just reabsorbed a whole bunch of water. We did most of our reabsorption in the maximal convoluted tubule, no question, but holy water reabsorption, we can totally do it. Now, I'm not drawing it in here yet, but where did the water go? I'll let you think about that. The water is going to get picked up by the bloodstream. And I'm going to show you how that happens, too, because it's phenomenal. Shall we see what happens in the ascending loop of Henley? Yes, indeed, we shall.