 Who's in charge of this thing? I am. So therefore, I get to decide what we're going to talk about next. I just modified the outline just a little bit and we're talking about vasopressin next because you know what? Vasopressin is made by the hypothalamus. Do you remember this? Vasopressin is also known as anti-diuretic hormone. It's produced in the hypothalamus. It's stored in posterior pituitary. So does it make perfect sense that osmoreceptors that send the message, send the action potential to the hypothalamus saying, dude, we're too concentrated here. Life is getting a little sketchy. Why don't you produce some vasopressin? Hypothalamus says, yo, post pit. Hook us up with some vasopressin. And post pit says, I got this. So now post pit dumps vasopressin into the blood. Are you ready for the scene? Allow me to set the scene for you. Here it comes. Here's the blood. Oh, bloody hell. This is the blood. And guess who this is? This is vasopressin. Vasopressin or anti-diuretic hormone, A-D-H. Thank you, post pit. Let's just make a little note that post pit is the one who dumped vasopressin in because of the osmoreceptors that fired the message saying, dude, we're not stretching enough. So here comes vasopressin. All right. We are going to have filtrate. We're going to have nephrom. Let's just throw this into the collecting duct. This can happen in the collecting duct or the distal convoluted tubule. But are you in agreement that I could actually draw, okay, this is a tubule? And we know that tubules are lined with cells. So let's just call this the collecting duct. And here's the cells that are lining with the hot nuclei. And you're good, right? You can totally imagine this whole thing, right? So this is my lumen. And what's in there? Filtrate. And who just arrived on the scene? A-D-H, anti-diuretic hormone. Okay. Receptors on the outside of cells in the collecting duct and the distal convoluted tubule, okay? Those receptors are available. Now watch what happens. A-D-H diffuses in. Binds to the receptor. When it binds. So that's the first thing that's going to happen. I'm going to draw you a picture and tell you about it, and then I think that somewhere I should write down what happens. When it binds, it's going to stimulate the exocytosis of a, this is a vesicle, and it's going to be gigantic because my cells aren't big enough. This is just a bubble of cell membrane. But guess what's inside it? Embedded in this vesicle of cell membrane is a water pore. That thing is a channel. It's called an aquaporon. Aquaporon, okay, it's not poron. It's aquaporin, porin. And it's basically just a water channel. If aquaporins are not in the cell membrane walls, then collecting duct and distal convoluted tubule cells are impermeable to water, just like the ascending loop of Henle. But we can make the collecting duct and the distal convoluted tubule permeable to water. We can make it happen. We can go rogue. Say, dude, it's time to get permeable to water by throwing in some vasopressin which causes exocytosis of these aquaporin-containing vesicles. So watch what's going to happen. If you exocytose this, if you just send it over, you exocytose the vesicle that contains the aquaporin. Do you agree? Can you visualize? The end result is that we're going to end up with a water pore embedded on the luminal edge of the collecting duct cell. Now, what's going to happen next? Well, remember that when our filtrate enters the scene, it's probably got a concentration of, let's just say, 100 milliosmoles. And this right here, the collecting duct is sitting inside that medullary concentration gradient. So at the very least, this is 300 milliosmoles. And it's getting more concentrated as we go down. So let's say this is 500, this is 700, this is 1200 down at the bottom, right? That's not a surprise because this whole thing is sitting inside that crazy concentration gradient. Throw some water pours into the mix. Dudes, water is going to be sucked out. What happens when the water gets sucked out? What's going to happen to the concentration of the filtrate if you remove water? The filtrate is going to become more concentrated. Do you agree? We'll get up to 300, we'll get up to 500. And how many aquaporins you insert into the luminal edge? You can get all the way up to 1200. If you can get your peepee up to 1200 milliosmoles, that's the orange peepee that you're like, oops, didn't drink enough water today. You probably don't feel so hot either. But you can do it. And then what happens to all that water? It goes back into the blood and then it goes back to the hypothalamus and you just increased the volume of water in the blood which is going to cause those little osmoreceptors to stretch which means they're going to send the message to the hypothalamus saying, dog, you did your job, we're cool. No need to do anything. Hypothalamus says, I'm cool. That's a little bit of negative feedback, yo. And we're done. That's the story. That's how vasopressin works. What questions do you have for me? Nothing, right? Now are you ready to talk about aldosterone? Okay, I guess it's time to talk about aldosterone. Aldosterone acts somewhere else and it has a totally different mechanism, a totally different like sensory receptor that stimulates the production of it. So that's why it's better for us to talk about it right now.