Which of the following conditions does not contribute to the creation and/or maintenance of the medullary osmotic gradient?

a. The countercurrent exchanger of the vasa recta

b. The countercurrent multiplier of the nephron loops of cortical nephrons

c. The countercurrent multiplier of the nephron loops of juxtamedullary nephrons

d. The permeability of the medullary collecting system to urea and other ions

Video transcript

Hi, everyone. Welcome back. Our next question says in the Rhino medulla, what is the main function of the vas erecta? A filtration of blood b, maintenance of the osmotic gradient c reabsorption of water and electrolytes or d production of concentrated urine. So, first off, we should recall what is the vasa? And they are the long straight capillaries that parallel the long nephron loops of juxtamedullary nephrons. So first off, they're capillaries, they're carrying blood, not the tubular fluid or the filtrate within the kidney. Always important in the kidney to remember what's carrying what liquid as we recall. When we think about those long nephron loops, what is kind of the salient feature or most important thing going on? And we recall, we have this osmotic gradient set up in the medulla and within those tubules where you have the concentration of the filtrate within the tubules and of the surrounding peritubular fluid that is increasing in concentration as you go down the loop. So concentration increases as you go down and that's established because water really diffuses out of that thin descending limb, but it's fairly impermeable to solus. So it's becoming more concentrated as you go down the loop down to the bottom where it's most concentrated. And then this thick ascending lamb is fairly impermeable to water. It's also impermeable to solutes. But you have active transport of sodium and chloride. So I have these double arrows here showing going out into the space ions out of the tubule into the peritubular fluid. So the fluid inside the tubule is becoming more dilute the peritubular fluid becoming more concentrated there. But to maintain this whole entire gradient, you need to have something that carries away excess water and solus that would disrupt the gradient. And how conveniently we have this, these capillaries in the area around these tubules. Let's draw my spacing isn't very accurate here, but I'll put VR in these little nor is my sizing. Please ignore that. So we have our, our loop, the peritubular fluid and then we have the vas rector running along here which can absorb water and sos and carry them away. So that main function here is choice B maintenance of the osmotic gradient. When we look at our other choices, we have choice, a filtration of blood. But this is primarily a function back earlier in the glomerulus where you have that filtration membrane system with your fenestrated um endothelium, you have your basement membrane. The pot toys that whole setup that is filtering blood back earlier in the glome. So choice A is incorrect. Choice C reabsorption of water and electrolytes. Well, this does occur in the proximal convoluted tubule as we can see. But this is not the main function of this portion. The main function here is the establishment and maintenance of this gradient because that's going to allow the concentration of the urine when you get further along in the collecting ducts. So it establishes that you have this dilute, relatively dilute filtrate coming into the collecting ducts at the end of this loop. But it's concentrated with urea and other waste products, mainly that you want to be carried out of the body. And you've reabsorbed all the useful things then that concentrated filtrate or excuse me, I should say the less concentrated filtrate that's coming out of this loop can then go as it goes into the DC T and the collecting ducts, those aquaporin that are present, there can allow passage of water out of that area. And now you have this concentrated urine that could be produced. So the whole purpose of this gradient that's set up is to then allow the production of concentrated urine later on. So we see that's our choice d production of concentrated urine. And we would say that the vas erecta participates or makes this possible, but it's not what produces the urine itself. Those these are the vas as the capillaries, this has to do with movement of blood, not production of urine. So it's an indirect involvement but not our answer. And again, we sort of return to our discussion. We started on of choice C re absorption of water and electrolytes that's happening from this proximal convoluted tubule happening along here. But again, we're talking about the function of the vas reca its main function in this area is carrying away that excess water. And so that's going to maintain the osmotic gradient. So choice C not our main function, which is what we're looking for. So the main function of the vasa is choice B maintenance of the osmotic gradient. See you in the next video.