 Are you ready to have your mind blown? Of course you are. Your kidney is phenomenal. Look at this. Okay, we know that in the interstitial fluid, we've got this crazy concentration gradient. And the question is, as we're moving solutes in and out and as we're moving water in and out and changing the concentrations of the filtrate, how do we pick that up in the bloodstream? And it has to do with this amazing setup called a countercurrent exchanger. So watch. We know that the filtrate is coming in at 300 milliosmoles. And you can see that I'm back to our original drawing. I've got the fluid in the proximal convoluted tubule and it heads down into the descending loop of Henley. As it heads down, we know in the descending loop of Henley, we're going to send water out. That's going to concentrate the filtrate. The filtrate is going to get more and more concentrated. And we've done this, so you're cool that we keep getting water out the descending loop of Henley. It's going to keep coming out and out and out until we reach the base where now we've gotten rid of so much water. Now that water has gone into the interstitial space. Now we're going to pump solutes out. So I'm telling you that our solutes are getting pumped out now. Now here's the cool part. As we pump out our solutes, you're awesome and accept the fact that we keep pumping until we get, like, what, 100, whatever. Here's the amazing thing. Our blood also has an osmolarity. Do you agree with that? It's going to be 300. But look at what happens. Look at who the blood goes by first. The blood is passing by right next to the ascending loop of Henley. So literally, the solutes that come out go into the ascending, the blood supply. And there's a concentration gradient because now all this stuff is coming out and it's going to be picked up by this lower concentration blood that's flowing in. So the concentration of the blood is increasing as it goes down because it's going right next to the ascending loop of Henley and it's picking up all those solutes that are being pumped out. So does that work for you? Can you imagine how that is the case? What happens if the blood picks up the solutes and takes it out of the interstitial fluid? We now don't have that 200 gradient anymore so we just pump more solutes out. So this is really a pretty endless situation where we can continue to pick up solutes until literally we reach 1200. Now this is another incredible thing that happens. As we ascend, now what was coming out of this side? Doggies of a feather, it was water, seriously. So as we ascend in the backward direction and it's a counter-current exchanger, water is going to come out, we're going to keep going up and pick up the water. So as the water gets picked up, our concentration is going to decrease until we get back out of the madness at the concentration that we started. Meanwhile, we picked up solutes. We didn't lose anything. All we did here was grab stuff. We grabbed solutes on this side and we grabbed water on this side. And by grabbing solutes first, we were able to pull them all out and by grabbing water second, we were able to pull it all out and take it back into the blood supply. Holy, does that blow your mind? That is how we ended up filtering out 180 liters of fluid and then reabsorbing all but 1.5 liters of Pp. That's unbelievable. We're going to use our medullary concentration gradient in the next section when we talk about, in the next lecture, when we talk about how hormones help us maintain osmotic equilibrium. But we're not going to deal again with how it's maintained or how it is established. So it would not surprise me in the slightest if you needed to go mess with this again and again. I can't tell you how many hours I've spent messing with this. It's definitely a tricky concept. So spend some time. Share any resources that you find that are awesome. And kidneys part two, catching kidneys part three, coming up.