 I must draw you a picture. First of all, this is the lumen of my nephron. You have to know where my lumen is, because otherwise none of this is going to make sense. This is the lumen in the proximal convoluted tubule. So what do we know about proximal convoluted tubule cells? We know that they have a high surface area full of microvilli on the luminal end. I'm not drawing all that stuff on there. We also know that ultimately, if we are going to reabsorb something, we want to send it into the blood. So we're going to have to cross the interstitial space here and get into the blood. That's reabsorption. So what was our goal? We're talking about since we're in the proximal convoluted tubule, we're talking about the pound of baking soda that you just filtered out of your blood. And we've talked about how important pH homeostasis is, and I'm telling you right now, dude, we better not excrete all of that. So I need to erase the lumen thing, because I want you to remember that it's there, but I need that space to draw things. Look at me erasing. See me erase. Okay, I'm done. But I still want you to remember that this is the lumen. I just need to be able to draw all my transporters. Oh my gosh. Bicarbonate ions, HCO3 negative. Here they are, a pound of them. I'm going to begin with a transporter in my proximal convoluted tubule cell that exchanges, are you ready for this? Sodium, four, are you ready for this? Hydrogen ions. What? Okay. So we just secreted hydrogen ions. You might be like, dude, is that really a great idea? I mean, this is the mechanism that we're going to use to get a pound of baking soda back into our bodies. So if we're exchanging hydrogen ions, we're going to be excreting a lot of an important substance, and that's going to mess with our pH. So we don't want to leave that hydrogen ion out there. Guess what? There's a player that you know, you know this guy, embedded in the cell membrane. Here's somebody I'm going to give you a hint. His initials are CA, and I'll write it over here. That's my friend, you've seen him before. Carbonic anhydrase. Do you remember carbonic anhydrase? Carbonic anhydrase was that enzyme that can take bicarbonate ions and hydrogen ions to facilitate that crazy chemical reaction that's really famous, what will they turn into? They will turn into carbon dioxide and water. What? Seriously? Seriously. What happens to carbon dioxide and water? It's going to diffuse into the cell. Both of them are going to diffuse into the cell. And guess who's inside the cell? You know it's true. Old boy, carbonic anhydrase is in here. What? And turns carbon dioxide and water into, who would you expect? Hydrogen ions and... Okay, I can't do the hydrogen in that color because I have to make it red. Bicarbonate ions? Seriously. And hydrogen ions. True story. What happens to the hydrogen ion? Awesome. It's going to go and get exchanged and come back in and go and get exchanged and come back in. And meanwhile, what do you think is going to happen with the bicarb? Well, this is just every bit of magic. There's another transporter that co-transports sodium. Let's go down your concentration gradient sodium and bicarb out and into the blood. Done. Did you follow that? The bizarre thing about this little setup is that carbonic anhydrase facilitates a chemical reaction outside in the lumen. It's like embedded in the cell membrane. And then it functions again except it does it in the other direction. I'm sure that there is something awesome that we could explain for how it does that, but it does that. And by doing that, we've just absorbed sodium, which of course we want to absorb that as well, and bicarbonate ions. And we've recycled these hydrogen ions, which is like what a great setup. This is how we absorb a pound of baking soda. This is not fine-tuning. This is don't die because if you pee out a pound of baking soda, you're going to be a sad story. Now, how do we fine-tune it? The fine-tuning is going to happen with intercalated cells? Ha. I mean, with intercalated cells in the collecting duct and the distal convoluted tubule. I'll be right back to tell you all about that.