 We're going to start in the proximal convoluted tubule. Now, it is absolutely critical that you accept and understand that I'm going to draw you a picture of a cell that has a luminal edge and a basolateral edge and that I'm going to have to bring stuff out of the filtrate and into the interstitial space. Ultimately, by the end of this lecture, I am hoping you will have a very good sense for how that stuff gets picked up by the blood. What's the process by which the stuff then moves into the blood? I'm hoping that that will ultimately become clear. I'm going to draw you a picture of a cell in the proximal convoluted tubule because I think I already said this, 70% of reabsorption takes place here, 70% of all the stuff that you filter out. So whatever 70% of 180 is, leaders, that's how many leaders are reabsorbed in the proximal convoluted tubule. This is wild. That's a lot of stuff to reabsorb. We reabsorb it into the interstitial space first, and then we have to reabsorb it into the bloodstream. If you don't reabsorb it into the bloodstream, it's pointless. So here's my picture. I'm taking a cell, one single cell from this edge of the proximal convoluted tubule so that you can keep track of where my lumen is because it makes a difference. Cells range from simple cuboidal to simple columnar. So this is a single proximal convoluted tubule cell. This is the lumen of the tubule. This is the interstitial space. It's simple cuboidal. Let's just call it cuboidal. It's simple cuboidal epithelium. So there's a basement membrane. And then this is all interstitial space out here. We have a nucleus. That's important. And what did you notice? What did I just draw up on the top? Holy microvilli. Why? Because that, the presence of microvilli on the lumenal edge of my proximal convoluted tubule cells increases surface area to allow reabsorption to take place. We're going to be reabsorbing 70% of our stuff. The more surface area we have to pull it off, the better off we're going to be. Now, reabsorption. We got to get some solutes out of the filtrate. Don't forget that this is filtrate out here. So if we want to get solutes out of there, I'm going to give you an example of how this might possibly work. I were to throw a transporter on here. This is my transporter friend, somebody that we've seen before. This little guy is sigilte. Do you remember sigilte? Sodium glucose transporter. This was an example of a transporter that can bring in sodium. Sodium actually comes down its concentration gradient. So right here, by putting sigilte in the lumenal edge of the cell, we're able to pull sodium into the cell. Now, if you remember, the sodium, it moves down its own concentration gradient. So we better throw somebody in here right now to deal with the fact that if we allow sodium to stay in the cell, we're going to build up a high concentration of sodium inside. So we need to get rid of the sodium. How are we going to do that? I know you can do this. Dude, how about this guy? This is my favorite. Who's that guy? That's the sodium potassium pump. So look, let's take the sodium potassium pump. So pump actively, use energy to pump sodium out in exchange for what? Well, we're going to have to deal with potassium. We're going to bring potassium in. Now, what would happen if we allowed too much potassium to end up inside the cell? If you end up with too much, then sodium potassium pump is going to stop working. So you know what else we're going to throw in this cell? We're going to do some open potassium channels. And you know what? We're going to let potassium... We're not going for any kind of membrane potential here. All we're trying to do is get stuff from the filtrate into the interstitial space. That's our only goal. So, yeah, let's just let the potassium come back through an open potassium channel. You now have moved the sodium all the way through. Let's go back to Siggled. What came through on sodium's concentration gradient? That's what I'm talking about, doggy dogs. Glucose is going to come in. Glucose. Why should we care about glucose? Because glucose seriously is like gold as far as our bodies are concerned. Glucose is the easiest thing to metabolize. Cellular respiration, done. Definitely do not want to be peeing out glucose. If we are, we got some issue going on and we got to figure out what it is. If we end up building up too much glucose inside the cell, we don't want glucose in our cell. We want to get the glucose into our bloodstream so we can go to the other cells and be stored. Glucose is going to come out through another one of our transporters that we've seen before. Who's this guy? Glucose is an easy transporter, an easy transition. This is glut. Do you remember that guy? Look here, I'm writing it right here. Glut. Glut is going to allow, it's just facilitated diffusion, piece of cake, it's going to allow glucose out through its own concentration gradient. Sodium concentration gradient allowed us to bring glucose in. Now glucose is going to be like, dude, get me out of here. Go down glut and get out, no problem. We just absorbed sodium and glucose by setting up our transporters in this fashion. We absorb all sorts of things. Tell me something that's going to come through as we reabsorb sodium, as we reabsorb glucose. What else, what's going to follow inevitably as we remove solutes from the filtrate? Hopefully, you're like, dude, of course, water is going to follow the solutes. Did you think osmosis left us? Never. I hope that this is intuitive and makes sense to you, that yes, if we remove solutes from the filtrate, we're going to decrease the osmolarity of the filtrate, which means water, if it can, is going to move into the cell and then ultimately water is going to follow all the solutes that are going this way as well. The volume, the 70% volume, we're absorbing all sorts of particles. In fact, in a 24-hour period, we absorb an entire box of baking soda, the volume bicarbonate ions, seriously, a pound of baking soda is reabsorbed in the proximal convoluted tubule. If you're absorbing a pound of baking soda, water will come with it. Unless we set it up to have a waterproof wall, which we don't, water is going to follow all of these solutes. This is just one example of a mechanism to absorb solutes. We could look up a whole bunch of different solutes that we can reabsorb and I'm sure we know all sorts of mechanisms for reabsorption. Sometimes you hit a limit to what you can reabsorb and that's called the transport maximum and that's what we're going to talk about next.