 the amount of filtrate, the amount of particles, solutes that can be reabsorbed into the bloodstream is affected by the number of transport molecules available to do the transporting. Something like water, dude, water doesn't need a transporter. Water just goes directly through the cell membrane. So who cares? We're going to absorb as much water as our concentration gradients, our osmolarities will allow. But something like glucose, we can actually overwhelm our transporters. And there's a concept called transport maximum that deals with this. So to illustrate this concept, I want to do a graph for you. And my graph is going to be, I'm going to graph like a whole bunch of things. I think four different concepts on this graph. So three, three. But on the y-axis, we're going to throw on a rate. And the rate is going to vary, depending on the line that I'm drawing. But all of my rates are going to be milligrams per milliliter. So if I'm talking about reabsorption rate, then I'm going to be talking about how many milligrams in this substance I'm reabsorbing per milliliter over a certain amount of time. We're going to compare that to plasma glucose concentration. And again, we're going to use glucose as our example because we just dealt with the sigilts and the gluts. But we could do this with any solute that we wanted to think about. The first rate I want us to contemplate is filtration rate. So I want you to speculate, oops, filtration rate. I want you to speculate on how do you think plasma glucose concentration, the amount of glucose in your blood, is related to filtration rate. So this is my filtration rate on this side. Do you think if I have low plasma glucose concentration, am I going to have higher low filtration rate for glucose? I'm going to have low filtration rate. And as my plasma concentration increases, my filtration rate is going to increase. And do you agree that literally it doesn't make sense? The glomerulus and Bowman's capsule, they're like, dude, I don't care what's in there. As long as it fits through the fenestrated capillaries and the filtration slits of the potocyte pedestals, then we'll take it all. So filtration rate is, there's no limit to filtration rate. The more glucose you have in your plasma, the more you're going to filter. Does that make sense? Perfectly clear. I want to do reabsorption rate next. Compared to plasma glucose concentration. And here's the thing. What does reabsorption rely on? Transporters. So for a while, if the amount of glucose in the plasma that is filtered out is matched, we reabsorb all of it. In fact, we reabsorb all of it until the plasma glucose concentration is getting higher as we move this way on our x-axis. There's a point at which now it's so high that we've hit transport maximum, TM. And now it doesn't matter how high the plasma glucose concentrations get. We can't reabsorb more glucose because we don't have enough transporters to pull it off. If I could do the magic, ah-ha. If I could do the magic, I would totally do some magic and make it so I can reabsorb even more glucose because who wants to pee glucose? Nobody does, especially not anybody who is starving. Our bodies were not designed to pee out glucose. Now, speaking of peeing out glucose, if you do not reabsorb glucose, what if we talked about excretion rate? Okay, I'm going to make it orange because that way you know that it's... We're talking about pee-pee now. Excretion rate. Have we ever excreted glucose? Never, unless suddenly we can no longer. We've hit transport maximum. Plasma glucose levels are so high that we've overwhelmed our transporters. We can't reabsorb that glucose so it stays in the filtrate. And if it stays in the filtrate, and we use of a feather, it will get peed out. We will excret it. Once we hit transport maximum, the higher the plasma glucose concentration, the higher the excretion rate for glucose in our pee-pee. Did you understand that? It's all because of the number of transporters that we have. Again, glucose is just an example, and we totally do this with many different solutes. All right, we made it through the proximal convoluted tubule. Hopefully, we've reabsorbed all of our stuff. We haven't overwhelmed our transporters. And now our task is to look at what's going to happen in the descending loop of Henley. Let's go there.