welcome back folks we're continuing our lecture series here on the renal system and uh just to give you an update I'm actually recording from my office at home and it is coming down in buckets outside so it's raining very very hard so you may hear the rain on the roof here on my window as I'm looking out the window here doing this recording but let's continue here with the renal system today's mini lecture or this mini lecture I should say is one of the most difficult Concepts to grasp with the renal system we are going to be talking excuse me my voice we are going to be talking about what is called the renal exchange mechanisms or the counter current multiplier this usually sends Shivers up the backs of of medical students because this is a complicated topic but this complicated topic actually is giving the basis for how this nephron and all of its exchange mechanisms are able to concentrate urine concentrate solutes that the body wants to be able to get rid of and yet not lose water not lose water so let's jump into this and see if we can get you to understand the basis associated with the counter current multiplier now in talking about this counter current multiplier I need you to remember some points and and I need you to write them down so if you're taking notes on this I'm hoping you're writing this down the first one and at this counter current multiplier this renal exchange mechanism is all based on trying to excrete solutes that we don't need and yet hanging on to water so we've got to concentrate those solutes pull out as much water as we possibly can so the body doesn't lose it and yet get rid of these excess solutes okay so that's that's our main focus to do that to do that the kidney and the nephrons that make up the kidney the vascular Network that is sitting around those nephrons and the interstitial area that's sitting in between the nephrons and the capillary Networks are trying to create a high osmotic pressure High osmotic pressure I'm going to try to remind you High osmotic pressures referring to an area that has a very high concentration of solutes and because of that high concentration of solutes it can draw water towards it remember that and so high osmotic pressure in the interstitial fluid in the spaces outside of the Nephron outside of the the capillary Network in between cells everybody got that to create this High osmotic pressure we're going to use all of those mechanisms that we talk about in Prior mini lectures we're going to use active transport of ions to move these solutes against their concentration gradients everybody got that try to move them create a high osmotic pressure in a particular area and then hopefully draw water towards it okay that's that's our three big points okay again we're trying to create a high interstitial fluid osmolary got that well let's see how this is actually done hmm in this first image trying to make this as simple as possible this image um I actually helped create many many many many many many years ago I was used in a couple of different textbooks and I'm bringing it here to try to hopefully help understand what's kind of going on with what is called the counter current multiplier this image is showing you a cartoonish image of a nephron proximal convoluted tubule to your upper left the loop of Henley as you can see here this Loop up into the distal convoluted tubule and then into the collecting duct everybody got that we're looking at just one Nephron this nephron and the way it sits in the kidney I want you to notice then up here this red line that's coming across the top above it the cortical area of the Nephron excuse me of the kitten and below it the medullary area of the kidney if you need to go back and look at our our first few mini lectures to remember where the cortex is in the medulla this is really important this is really important to remember the anatomy here the cortical area up on top here if you look at the area outside of the tubule this is the interstitial area okay here and then you can see here peritubular fluid or interstitial fluid out in this area you can see its osmolarity is sitting at about 300 million moles now again you're going to have to remember back the beginnings of this semester 300 million osmoles was a very important number that is kind of standard osmolarity for cells in our body inside of the cell 300 million osmoles it wants the outside to be 300 million osmoles and if you have that water is not going to move much the cells are going to be really happy with the water they have outside of the cell is going to be happy as well so here in the kidney sitting up the in the fluid of the cortical area of the kidney osmolarity is sitting at about 300 million osmoles I want you to notice something as we move into the medulla and get deeper and deeper into the medulla getting closer to the middle of the kidney where the renal pelvis was look at the osmolarity change going from 300 million osmoles to an incredibly High 1400 million osmoles that is literally a desert folks literally there is the concentration of solving which is so high there that if if cells can't literally live in that area unless there's some sort of help they have some sort of support around the surface that sells because water is being sucked from it so so severely all right so that's outside outside of the Nephron well that will influence what's going on inside of the Nephron how so let's follow our tubule fluid our what's the fluid that's been filtered from the glomerulus which would be way over to the left here as that fluid is moving through we'll say the proximal convoluted tubule and down into the loop of Henley remember at the beginning part of the loop of henle water is permeable across the membrane Now is it going to want to move yes it's going to want to move because solute concentration is high in the interstitial area water is going to want to move out to try to dilute that out are solutes permeable in this part of the loop of Henry they are not they are not solutes are not permeable so as water leaves to go out here the concentration of solutes inside gets higher and higher because water is leaving as we follow the fluid moving through the loop on the ascending Loop and moving up into the distal convoluted tubule remember water is not permeable water is not permeable but we have active Transporters in the membrane so solutes like sodium potassium here chloride and other solutes are getting pumped out and into the interstitial area that is going to cause the interstitial area to get higher in concentration higher in osmolarity so as this continues to pump solutes out and into the interstitial area the concentration of solutes gets higher and higher on the other end water is going to be able to be pulled out we are using solute active transport to increase the osmolarity in the interstitial area so that we can draw water out does that make sense folks that makes sense now this is a very interesting and amazing concept that the body has kind of developed in itself let's see how that may have developed in this first image in this first image at the top upper left you can see tubule our nephron here with our proximal convoluted tubule Loop of henle moving into the distal convoluted tubule again cortical area above medullary area Below in the beginning as the kidney was developing fluid and the osmolarity associated with that fluid was sitting at 300 million osmoles in the interstitial area it was also 300 milliosmoles again this is during the development of the kidney as the active Transporters in that ascending Loop and distal convoluted tubule developed they started pumping solutes out of the Nephron as they pump them out that's going to go into the middle component or into I should say the interstitial areas around the nephron as that happens the concentration of solutes gets higher that also means the concentration of solutes inside of the Nephron gets lower well as this concentration gets higher here it holds water from the descending Loop of henle over because the concentration is higher now this interstitial area is a vast area this is huge and the num and the amount of solutes in there is tremendous so even when water is pulled from the beginning parts of the Nephron here it's not going to change that interstitial area very much and so as we move to the third panel here you can see 400 million ounces still here the inside of the nephron has changed to 400 Milli ozmoles because it's pulled the water out over here to the interstitial area over on this end of the loop concentration is is getting lower because solutes are still being pumped out still being pumped out as that increases the interstitial area will keep increasing its solute concentration and keep driving fluids or water I should say from the other parts of the loop and this will continue over and over and over again until at some point the osmolarity will not be able to increase much farther that's why won't it increase any farther than 12 to 14 1400 millio osmoles that's the breaking point of the walls of the cells the walls the cells that make up the Nephron this it gets to a particular pressure point where water and solutes have come to some sort of equilibrium now it's at this point and so 1400 milliosmoles is about as high as it will go now in other species of animals like the desert kangaroo rat it's interstitial area can get up to 2600 millios moles pretty amazing the desert kangaroo rep may not see water for months so it needs to be able to conserve water inside of its body at an incredibly High rate so pretty fascinating kind of setup now this is the Nephron in relations to the interstitial area solute concentrations and the pulling of water everybody got that now we haven't looked at the end of the distal convoluted tubule and the collecting duct let's look at that right now now in this image distal convoluted tubule at the top and you can see the fluid that's sitting within that tubule well we've pulled out some of the solutes but water was impermeable to the distal convoluted tubules wall so if water can't move up that means it's still kind of diluting out whatever solutes we had left so it's sitting at about 100 million osmoles here as we move into the collecting duct you can see here the wall of the collecting duct can allow water to come across but only if particular hormones or signals are given to those cells to open up channels if that is the case because of the high osmolarity in the interstitial area this collecting deck literally we could pull out every almost every last drop of water if the body needed to if the body needed to that make sense folks but again for water to be permeable we have to have a signal we have to have hormonal regulation kind of controlling this because normally it is not if the wall is not permeable we will get rid of that urine we will get rid of that fluid I should say how does that sound folks how does that sound all right I want to show you one last image well before I get to that image again we have been looking at the nephron moving into the collecting duct and the interstitial area I want to look at the vascular Network here very very quickly if we look at the blood vessels that would be sitting around the nephron now I realize this is drawn in the same way as the nephron but this is actually looking at the blood flow so this is the Vaso recta which sits around the loop of henle and the collecting duct and these are the Peri uh the pericapillary blood vessels that are sitting up around the proximal and distal convoluted tubule it also is trying to manage solute and water concentrations however in the walls of the blood vessel there are no exchangers or carriers everything is done by passive diffusion well because there's a lot of solutes sitting in the interstitial area those solutes flow into the blood stream because the blood osmolarity would be sitting at 300 Milli osmoles as blood travels that way or excuse me as solutes travel in you can see the concentration of solutes getting higher and higher water is going to get sucked out and into parts of the interstitial area but as the concentration of solutes inside of the inside of the blood vessel gets higher water will get pulled back in this is all moving in a different direction than what is happening with the cap the excuse me with the Nephron so it's running encounter in the opposite direction of fluid flow and solute flow than what we see with the nephron itself that is why this is called the counter current multiplier that the vascular Network sitting around the nephron and I'm going to move back here moving over so those capillaries would be sitting around the nephrons here they're exchanging these are fluids moving in counter to each other using the interstitial area and being able to use those solutes to pull water and put it back into the bloodstream itself everybody got that put it back into the bloodstream now let's go to these major points again to create a very high osmotic pressure in the interstitial areas we had to use active transport of ions once we use those active transport mechanisms to pull those ions into the interstitial area and create that high osmotic pressure fluid would follow water would follow so we increased interstitial osmolarity water follower now if water stayed in the interstitial areas that really wouldn't help us a whole lot we're trying to get it back into the bloodstream and maintain blood volume that's where the arterioles and capillary systems that sit around the nephron are so important they're pulling up those solutes water is following into the capillaries in the medullary area holding it up and then being distributed out of the kidney and back into the rest of the body in this image this is a very simplistic image taken from a textbook many years ago showing you kind of an outline here of a kidney a renal pelvis sitting here the medullary area that we had talked about before and the cortical area and this is just trying to show you osmolarity so remember in the cortical area we would have the glomeruli of those nephrons the proximal convoluted tubules and distal convoluted tubules and the medulla area in the medulla area we would have the loops of henle and the distal convoluted tubules the distal convoluted tubules how are we doing folks all right in our last image kind of a review here we have the tubule and the loop extending downward again water permeable on the descending loops and I should say in the proximal convoluted tubule as well on the ascending loops and the distal convoluted tubule water is impermeable impermeable however we have active transporters pushing solutes like sodium chloride into the interstitial area creating an incredibly High highly osmotic area in the interstitial area water gets drawn out we hang on to water we hang on to water as we move over into the collecting duct the water the walls if provided a signal one of those signals being vasopressin they will open up channels and allow water to be sucked out into that very high osmotic pressured area of the interstitial fluid not that folks not that now this is also showing you urea this is how we concentrate urea urea not necessarily permeable along most of the Nephron itself especially as we move into the ascending Loop the distal convoluted tubule in the beginning parts of the collecting gut not permeable as we move into the ending of the collecting deck nine times out of ten urea is not going to be permeable we're going to get rid of urea however if vasopressin is present urea and water look very similar and have very similar sizes we're going to hang on to a little bit of urea however it will continue to keep concentrating it over and over and over again So eventually we will get rid of it this is the counter current multiplier I'm hoping you took notes and I'm hoping you have questions for me ask them in the lab please ask them and lab or come to office hours and let's talk we'll talk to you later folks