Alrighty. Let's begin talking about the reabsorption of sodium as well as other nutrients in the proximal tubule. And if you're wondering why sodium gets its name in the title versus, like, potassium or calcium, it's just because sodium is one of the most abundant substances that gets reabsorbed from the filtrate. So that's why it gets its name in the title there. And when I say nutrients, I'm going to be focusing a lot on glucose in this video just because glucose is one of the other most abundant substances that gets reabsorbed by the kidneys. So sodium and glucose are kind of the stars of this video.

Now, reabsorption in the proximal tubule can either be active, which just means that it uses ATP, or it can be passive, which just means that it doesn't use ATP. Now, sodium reabsorption specifically is driven mainly by active transport, and that is going to be primarily through the transcellular route or going right through those tubule cells. Despite this, mainly driving sodium through active transport, sodium and other nutrients can also travel via secondary active transport as well as passive transport. So we're going to go through some specific examples of all of those.

Now, focusing on primary active transport, we are going to have our old friend, the sodium-potassium pump. And gosh, I have missed her. I'm so happy to see her. And this sodium-potassium pump is going to be fairly abundant in the basolateral membrane. So, you guys remember that the basolateral membrane is the one closest to the interstitial base. We go A, apical, B, basolateral, kind of in order there. So we're going to have a whole bunch of sodium-potassium pumps on that basolateral membrane that are going to be actively transporting sodium ions into our interstitial base. You guys remember that our sodium-potassium pumps, we think of our little pumpkin, right? We pump K⁺ in and we toss all that Na⁺ out. So we're going to get, 3 Na⁺ getting chucked out of that valve as we're bringing in 2 K⁺. And so that is kind of the main way that sodium is going to be leaving our proximal tubule cell.

And so if you look down at our image here, just to kind of orient you to what we're looking at, we have our filtrate over here. All these little purple dots that you see represent sodium ions. So these are those prominent microvilli. You can see how that's really going to be increasing the surface area of this proximal tubule. We have our tubule cell here, our interstitial space, and then, of course, our capillary. And so you can see along this basolateral membrane here, we have this little sodium-potassium pump right there, And so that is going to be a form of active transport. K. So it's pumping all of our little purple sodium ions out into our interstitial space where they can then be reabsorbed by that capillary.

Now, you may be wondering, well, how did the sodium get into that tubule cell in the first place? And what a fantastic question. You are on the ball today. And that is going to be via secondary active transport. So what's happening here is that basically because this sodium-potassium pump is working so hard to get sodium out of these tubule cells, this sodium over here in the filtrate now has a pretty steep concentration gradient pulling it toward these tubule cells. Remember, sodium ions are introverted. They want to get out of this crowded filtrate and into this nice quiet tubule cell with no other sodium. And so these pumps establish this concentration gradient, and then sodium ions from the filtrate are going to get pulled into our tubule cell via co-transporters that also carry some kind of specific nutrient. So things like glucose or amino acids. And those are usually moving against their concentration gradient, but the energy that is created by that sodium concentration gradient is enough to pull both of them in. So if you look down at our image, you can see we have this, kind of blue oval right here on the apical membrane now, and that is a sodium-glucose co-transporter. What it's going to be doing is basically pulling in one of our little green glucose and one of our sodium, and they get pulled in simultaneously using the energy from that sodium's concentration gradient. And so that co-transporter is going to pull them both in and put them into that tubule cell. So that is our secondary active transport. I'm going to label that now.

Alright. And then finally, we also have passive transport happening. So this is when solutes travel from the tubule cell into the capillary passively. So either via simple diffusion, and that will typically just be lipid-soluble substances like some vitamins will travel that way, as well as facilitated diffusion. And that will be things like glucose as well as other nutrients as well as other ions even. So if we're thinking of facilitated diffusion, we're usually just thinking of like some kind of nutrient-specific channel that's going to be along that basolateral membrane. So here, we're looking at just a little glucose channel there on that basolateral membrane that will allow the glucose to leave the cell. So that is our passive transport.

So just to summarize what's happening here really briefly, we have some glucose and some sodium hanging out in our filtrate. Those are going to simultaneously get pulled into our tubule cell via secondary active transport through this sodium-glucose co-transporter. That's going to be using the energy of the concentration gradient of the sodium. So now they are both in our tubule cell, but they're going to take different routes to get out of it. So our sodium is going to get sucked out of the cell, via primary active transport with the sodium-potassium pump, and that will dump it into our interstitial space where it can then enter our capillary. And then our glucose will just travel through this tubule cell and then exit passively via this glucose channel, enter our interstitial space, and then enter our capillary from there. Alright. So that is our little summary of reabsorption of sodium and other nutrients in our proximal tubule, and I'll see you guys in our next one. Bye-bye.