Kidneys are the major organs of the excretory system, and they're heavily involved in osmoregulation. They actually have two layers. This outer layer we call the cortex and an inner layer known as the medulla. And it's this inner layer that's going to be the saltiest layer. And the kidney is basically mostly made up of these functional units called nephrons. And these are tube structures that transport filtrate, which we'll talk about more in a moment. And these tube structures are going to be surrounded by blood vessels. Now nephrons are going to use the active transport of solutes to create salty environments that will help these nephrons and the kidneys reabsorb lots of water from the filtrate. And as I said, there are two layers to the kidney. You have the cortex, which is the outer layer, and the medulla, which is the inner layer. And you actually will see two types of nephrons in kidneys. You'll have what are called cortical nephrons, like cortex, and these are the most common type of nephron. And basically, the tubules of these nephrons just don't extend that deeply into the medulla. See, here's the boundary between the cortex and the medulla, here is a nephron. Jump out of the way. And you can see that this nephron only extends a little bit into the medulla, whereas this other nephron is a juxtamedullary nephron, which is mostly responsible for maintaining that osmotic gradient for reabsorption. So, the cortical nephrons are there to filter the filtrate and create urine and reabsorb lots of stuff. The juxtamedullary nephrons are not as common because their job is more of a support role. They're just there to make sure that this area stays super salty, and that's why they extend very deeply into the medulla as opposed to the shallow extension of the cortical nephrons. And, hopefully, you can see in this image that we're basically over here, we're looking at like a slice of this. It almost as if you took, you know, a chunk out like that and zoomed in, that's what you'd see. And hopefully you can also see just how riddled with blood vessels the kidney is. It's super infused with blood vessels, and you'll see why momentarily this is so important. So with that, let's actually go ahead and flip the page.