 Hi, cute hearts. Today we're going to finish up our conversations about the kidney. And in particular, we've looked at the nephron, we've looked at the process of filtration, how the blood is filtered, 180 liters a day, into the nephron tubule. Then we've looked at reabsorption. How do we reabsorb most of that 180 liters and filter out every 24 hours? And we've got some specific mechanisms. We went through the whole process of establishing this interstitial medullary concentration gradient in the kidney. And now we're going to use all the tools that we've got together to look at how the kidney participates in maintaining homeostasis of the extracellular fluid. First of all, the kidney plays a huge role in maintaining concentration of the blood. So there are receptors, sensory receptors, in the hypothalamus that detect or monitor the blood concentrations of just stuff in the blood. And messages get sent from the hypothalamus to the kidney that act on the nephron, specifically on the distal convoluted tubule, which was this little wonky thing, and the collecting duct. There are two hormones that we're going to look at today that are going to help us modify the nature of the urine that is produced. You've got all the pieces for how we're going to make urine. Now we're going to fine tune the urine and make sure that what we're getting rid of is what we actually want to get rid of. Then we're going to look at water, how we can make sure we don't pee out too much water if we need it. Then we're also going to look at pH. This is the place where we're going to tie up some of the concepts that we've already been introduced to, such as hydrogen ions, carbon dioxide, the role of the respiratory system in maintaining pH homeostasis, and specifically now we're going to look at the role of the nephron in maintaining pH homeostasis. Most of everything that we're talking about today is taking place in the distal convoluted tubule and the collecting duct. There's a little bit of pH homeostasis stuff that we're going to talk about that happens in the proximal convoluted tubule. Otherwise, this should be a good adventure. Let's get started talking about how the body senses the osmolarity or concentration of the extracellular fluid.