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Countercurrent multiplication in the kidney

Created by Raja Narayan.

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Video transcript

Voiceover: So, we've already talked about this part of the nephron, right? Remember, it's called the glomerulus, the glomerulus, and it's the structure that receives an arteriole that's called the afferent, meaning going towards, arteriole, that's the arteriole that branches off of the renal artery, and it gives off this vessel right there. This is called the efferent arteriole, efferent meaning going away from, and these are all talking about the glomerulus. So, the afferent arteriole goes in, becomes really really squiggly, and then it comes off as a single vessel, the efferent arteriole. In this process, a ton of fluid is filtered out of the blood and goes into this yellow capsule right here. This is Bowman's capsule, Bowman's capsule, which is the first part of the nephron to actually collect fluid that's going to be filtered to become urine, and in this sense, the kidneys are pretty amazing. Do you know in desert rats, these are animals that may never see free water in their entire life, because they live in the desert, their kidneys are so good at concentrating urine and absorbing water that sometimes their pee can actually be solid crystal. That sounds pretty painful, but it works for the desert rodents. Let's figure out how our nephrons work for us. After the glomerulus, the next part of the nephron is this guy right here, which is pretty convoluted, wouldn't you agree, but it's close to the glomerulus, so, we name it according to those features. It's proximal to the glomerulus, so we call this a proximal, and because it's so windy, we call it convoluted. It's the proximal convoluted tubule, proximal convoluted tubule. Kind of a mouthful, but these words perfectly describe what it is. The proximal convoluted tubule is very important for reabsorbing ions, like sodium and chloride, but also some of our other builders of macromolecules, like amino acids and even glucose, and remember when we're absorbing things like this, and especially with sodium, we're going to take water with it as well. So, water is reabsorbed at the proximal convoluted tubule too. In fact, I've read somewhere that the proximal convoluted tubule reabsorbs about 65 percent of all of these important nutrients. That's the most of anywhere else in the nephron that we're going to talk about. So, it's pretty important. Where does the nephron go next? Well, it actually becomes this loop right here. It descends deep into the kidney, and then it ascends back up again. This entire structure is affectionately called the loop of Henle, the loop of Henle, and it's got two limbs to it, as I've shown here, and they're going in opposite directions. One is called the descending limb, descending because it's diving deep into the kidney, so this is the descending limb, and the other part of it, this guy right here, is called the ascending limb, ascending because it rises up, ascending limb, and the reason why this is significant is because remember the kidney is kind of broken up into two main overall parts, right? There's a part that we call the renal cortex, it's above here, and then there's a part we call the renal medulla, which is down here, and the thing to remember is that the renal medulla is very salty, very salty because we have a lot of ion reabsorption happening here. Now, I should further specify that the descending limb and the ascending limb of the loop of Henle reabsorb very separate things. The descending limb reabsorbs water, so we have mainly water coming here, and, in fact, there are no ions that are reabsorbed at this point. It is impermeable to ions. On the other hand, the ascending limb does the exact opposite. Here, we reabsorb things like sodium, chloride, potassium, and, in fact, this part is impermeable to water. No water will be reabsorbed here in the ascending limb, and because of this, we have a very beautiful system that occurs as a result. This is called countercurrent multiplication, countercurrent multiplication, which is also, I'll admit, a mouthful, but it completely makes sense, I promise. Countercurrent multiplication. We say countercurrent because the descending limb and the ascending limb go in opposite directions. That's why it's countercurrent. Multiplication means that when we reabsorb ions in the ascending limb here and make the medulla salty by not reabsorbing water, that drives water to be reabsorbed passively in the descending limb, and we have a video that goes into detail about transport processes in the nephron, but here just remember that water is reabsorbed passively, so no energy is expended to reabsorb water, and this is because we have used energy here in the ascending limb to reabsorb these ions. So, active transport is used here, and by actively pumping ions into the medulla, and no water in the ascending limb to make it salty, we can multiply the amount of water that is reabsorbed passively, because it's driven into this space around the tubule or the nephron. The space around the tubule is just called interstitium. I'll write that off right here. So, this is the interstitium. This is anything that is not the tubule or vessels, and that's just space around here. This is all just interstitium, just hanging out right here, and so all this ions that are reabsorbed into the medullary interstitium down here, the interstitium of the medulla, drive the passive reabsorption of water. All right, so I think we have a pretty good understanding of the loop of Henle and the countercurrent multiplication process that happens here. The next part of the nephron is this guy, that kind of loops back and just kisses the glomerulus right there, and I'm sure you have astutely noted that just like the proximal convoluted tubule this tubule is also certainly very convoluted. So, it's a tubule that is convoluted, let's say, but it's not as close to the glomerulus as the proximal convoluted tubule was. So, instead, we shall call this guy the distal convoluted tubule, and this dude is responsible for the reabsorption of other ions, like sodium and chloride, and it helps to just pick up more of these important nutrients that we'd like to have in our bloodstream, that we don't want to pee away. The other thing I should mention now, that I promise we go into more depth in in a separate video, is this very scientific kiss that happens here, when the distal convoluted tubule comes by the glomerulus again. This produces something that's called, and this is a mouthful, the juxtaglomerular, juxtaglomerular apparatus, juxtaglomerular apparatus, and all this is responsible for is to control blood pressure. This is part of the kidney that's used to control blood pressure, and we'll talk about this in detail in a separate video. So, now that the distal convoluted tubule's come up here and kissed the glomerulus and kind of come out here, it's time to collect whatever leftover fluid we have, and we do so in this guy right here. This is called the collecting tubule or the collecting duct. So, it collects things that we have left over in the lumen, or inside of this nephron, and one thing to note is that there are many DCT's, or distal convoluted tubules, that feed into this single collecting duct. So, there is a DCT, there is a DCT, and there is another one down here, right? And we actually reabsorb a couple of things in the collecting tubule as well. The main thing that we reabsorb into our interstitium is water, and another thing we reabsorb, that I'll show deep in the medulla right here, is urea. Urea is one of the main waste components that we actually pee away, but sometimes the kidneys like to hold onto urea to increase the osmolarity in the medulla, to help drive water reabsorption in the loop of Henle. This goes into a process that's called urea recycling, if you've heard of that term before, but we're not going to go into detail for right now on that. Instead, we'll just mention here that urea is reabsorbed to maintain osmolarity, maintain the osmolarity in our medulla that will help drive water reabsorption in the loop of Henle. And, finally, I want to close the loop on what happens to this poor efferent arteriole right here, because we came off the afferent arteriole, and this I promised you would turn into a capillary and then a venule, and here's where I'm going to talk about that, because we've reabsorbed all of these awesome things here in blue, but we don't have a way to put them into the bloodstream yet. Well, the efferent arteriole gives us a way to do that, and it does so by kind of coming off this way, and just like all good arterioles, it branches off into even smaller branches, so much so that we branch off into smaller capillaries, and these capillaries will dance across our nephron and collect all this good stuff that we've talked about, here in blue, that gets reabsorbed into our interstitium, and I should mention that because these capillaries kind of hang out all over the place, where our tubules are, we say that they are peritubular, peritubular meaning just around the tubule, so, sure enough, their official name is peritubular capillary, or we call them all peritubular capillaries, and so after we've collected our nutrients in these peritubular capillaries, we come back here, where we then start doing the exact opposite, because now we've lost our oxygen but we've reabsorbed these nutrients into our bloodstream, and then this will kind of come back together and head off into, I think you guessed it, the renal vein, and the renal vein will then take this back to the rest of the body, and that leaves the rest of what we've collected here in our tubules, in our lumen right here, that goes away, and this is going to become our urine. Our collecting tubule is where we first have urine that's going to be sent off into our renal calyces and then further on to be peed away.