36: Renal Physiology IV

Transcribed by Leslie Afable 4/18/2013

Organ Systems Lecture 36 – Renal Physiology IV by Dr. Joel D. Schiff

Hypertonic Medullary Interstitial TissueStudent – Asks a question that cannot be heard.Dr. Joel Schiff – It will do it except this doesn’t have a direct mechanism to do it other than it doesn’t stimulate secretion of aldosterone. The major effect on blood volume is through aldosterone and it’s used in situations where you want to raise the GFR. So that’s how you regulate the GFR to stay at 180 and because that’s a homeostatic mechanism, it keeps things working the way they do. One of the things they do is to add solute to the medulla to keep the medulla hypertonic.

Factors That Threaten the Hypertonic Medulla (in addition to diffusion)Dr. Joel Schiff – Now there are a couple of things you have to keep track of in terms of the medulla. One, here you have 60L per day of filtrate (PCT) and you go down to the hairpin turn through the descending limb of the LOH and you’re down to about 36. So that means that 24L/day of water was osmotically sucked out of the filtrate into the interstitial space. Assuming that you eventually at the end of the DCT and collecting tubule you have your 18L/day of filtrate reaching there, assuming you have adequate supplies of ADH and so on, so that there’s osmotic reabsorption in the collecting duct. From the collecting duct you’re reducing 18L to 2L per day, that’s your urine output. There's another 16L/day of water being sucked into the interstitial space in the medulla. You’re ascending limb of the LOH is doing a lot of work dumping solute in there, but meanwhile how big is the total volume of the medulla? You certainly can’t dump 16 +24 is 40L/day of water into the space in the kidneys. There has to be a way of removing it. But there's a proviso here. You want to be able to remove the water, this 40L/day, without taking away all the solute if you can. You can’t avoid.. if you’re removing water, you can’t avoid taking away some solute which of course increases the burden on the ascending limb of the LOH to put the solute there. But what you can do is minimize the removal of water. The

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circulation.. ..the other thing of course is this ascending limb of LOH is doing a lot of work. You’ve got lots of mitochondria driving those sodium pumps. What do mitochondria need? You need metabolites, you need glucose eventually being broken down into pyruvate, and you need oxygen which means you need a circulatory system that supplies everything down into the medulla. There have to be capillaries going down into the medulla and coming back up or going through it or somehow to bring the nutrients and bring the oxygen and take away this 40L/day of water.

Peritubular Capillaries Send Off Vasa RectaDr. Joel Schiff –What we have evolved is a specialized group of capillaries that are referred to as VASA RECTA. Up in the cortex you have your peritubular capillary bed that sort of runs all around here. It originates from the EFFERENT arteriole, remember. These are PERITUBULAR CAPILLARIES. And what they do is they put out little branches that go down into the medulla and turn around and come back up and eventually rejoin the capillary bed. There’s very little branching here. Remember most capillaries appear in form of a sort of network. But these don’t branch, don’t bridge, don’t connect to each other, only minimally. For the most part they loop down and loop up and they’re called VASA RECTA. And what do they do? Well they bring blood, obviously because you need the oxygen. So what happens is this, you’ve got these long straight capillaries going down and right next to them, the other branch of that capillary, it sort of echoes the LOH in terms of the nephron except on narrower, skinnier scale. And what you have got is a very slow movement of blood going down into the medulla and then coming back up. These are small capillaries. Dr. Kinally has described capillaries to you. I don’t know if she emphasized this point. How big is a capillary? Compared to how big is a red blood cell (RBC)? Not the same, that’s the whole point. RBCs are bigger than the diameter of a capillary, at least in humans. The only way the RBC can fit through a lot of capillaries is by folding it in half. That, by the way, is the reason for a lot of peripheral vascular problems in people who have sickle cell disease. RBCs that have the Hb-S, the sickle gene, don’t fold in half very well. So you’ve got this very low blood flow going down the vasa recta and it turns around and comes back up. So it’s doing some of the job, it’s bringing oxygen to the depths. It’s taking away a certain amount.. and the plasma inside these capillaries as you go down is equilibrating

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with the tissue surrounding it. So the osmolality of the plasma as it goes down is going up towards1200. There is solute moving in but, more importantly, there is water being sucked out.

Dr. Joel Schiff –As a key here, if you just have fluid going through a narrow pipe and you were sucking water out of it the way you do in the filtrate, you could really remove a lot of water. Here you’ve got a sort of framework in this capillary. Here is the capillary and here are your folded RBCs. This provides a framework you can’t really suck a lot of the water out from the plasma because the RBCs are not letting the capillaries constrict completely. So they are providing a sort of framework that keeps these capillaries open and limits the amount of water that can be removed from the capillaries into the interstitial space on the way down. But now when you’ve got whatever plasma in there it is very hypertonic, after the turn, when the vasa recta go back up, this hypertonic plasma in here on the way up is going to be osmotically sucking water into it. So typical vasa recta, little capillary loop, effectively leaves the medulla carrying about 4 times as much water as it came in with. But with a minimal amount of solute or of extra solute compared to what it came in with. So you’re doing a relatively good job of removing those 40L per day when you have a lot of vasa recta loops. You are trying to remove that 40L/day of water with minimal removal of solute. So basically this is the way the kidney is able to supply metabolites to the depths of the medulla without further washout of the solute.

Dr. Joel Schiff –Ok, so you more or less have gotten a picture of what’s going on in the kidney. For the next whatever time.. ..speaking of time, do you want to take a little break? I see signs of enthusiasm for that. Ok so take 5-10 minutes or whatever.

V. Removal of Nitrogen-Containing WasteDr. Joel Schiff –Ok should we get back to work? From here on we are going to be discussing a number of different topics that relate to the kidneys and they’re not in any particular order but everything has to come together ultimately. One of the main functions of the kidneys is to help your body to get rid of the garbage. And a significant part of the garbage that it has to get rid of is nitrogen-containing waste products because we metabolize proteins into amino acids and amino acids have that amino thing on them and then we take in, at least in our current diet, a lot more protein than we actually use for construction for building proteins. We have this nitrogen-containing waste that we have to get rid of. So from metabolizing proteins we also breakdown amino-acids and deaminate them and the liver does a lot of that in gluconeogenesis to generate sugars from the amino acids. So we have a lot of

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these nitrogen-containing compounds and one of the things for getting rid of them, through the kidney that is, is that they have to be in the form of water soluble compounds. Now there are a number of candidate water soluble compounds that contain nitrogen, one of which is ammonia or ammonium ions. One of which is uric acids or any of the salts of uric acid, the urates. One of them is UREA which is a small molecule that contains nitrogen and it is highly water soluble. It has some interesting properties which I will get into a little bit later. We don’t use uric acid as a form of getting rid of nitrogen. Birds do as you’ve ever parked your car under a tree, you’ll discover that’s uric acid. Because the thing is we, standing on the ground, can handle a lot of extra weight of water. We have a lot of water in our bodies. Birds to be able to fly, kind of have to minimize how much weight of water they have to carry around. So they don’t produce a liquid urine, they produce this sludge which contains uric acid as their way to get rid of nitrogen.

Dr. Joel Schiff –If we have excess uric acid in our bodies, crystals form because it’s not very soluble in water and it’s not very soluble in plasma and we end up depositing crystals and that is known as GOUT. From what I understand, it’s rather painful to have these uric acid crystals forming in various joints. The classic, of course, is the large toe. So we don’t use uric acid as a way of getting rid of nitrogen-containing waste. We don’t use ammonia or ammonium ions either because they satisfy the solubility problem (they’re very water soluble, nothing can be more water soluble than ammonia or ammonium ions) the problem is that they’re HIGHLY TOXIC to a number of tissues including the nervous system. So you don’t want to have a lot of ammonia or ammonium ions in your circulation for the kidney to have to process because if there’s enough to make a significant contribution to getting rid of the nitrogen wastes, it’s going to be rather poisonous throughout your body.

Dr. Joel Schiff –So the main form in which we get rid of our nitrogen waste is UREA. Urea is a small molecule. If you look at some textbook-type diagrams of the kidneys and they show what causes the osmolality of the various parts of the medulla and so on and so forth, they’re going to show a lot of urea and their tables will show tons of urea throughout the kidney and they’ll try to attribute the HYPERtonicity of the medulla to urea but it’s NOT so. The reason for this is because urea functionally doesn’t have any osmotic effect at all. It’s a solute, yes, and the whole classic description of osmotic pressure is that you have say 2 compartments with some sort of membrane between them and you have a solution of water on one side of the barrier and you have water plus a solute on the other side. Therefore, in a sense if you look at it, the water concentration is higher here because you have just water and nothing else. The solute is actually diluting the water so there’s less water here so water tends to move toward that way (side with water + solute). That’s fine as long as the solute can’t move that way (in the other direction). But urea is freely permeable through all biological membranes. So it CAN’T contribute to osmotic pressure at all because it can’t draw water to it, in a sense, because it’s just as easy for urea to go the other way and it’s probably easier. But we do handle urea as the main form of getting rid of nitrogen waste because it’s very highly soluble in water

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and our urine contains, as it’s eventually released/excreted, has a lot of urea in it if we have been eating proteins.

Clearance Dr. Joel Schiff –One of the interesting concepts that came up early in the study of renal function is the need for, and I’m talking 120 something years ago.. we’re talking 1880s-1890s or thereabouts, where people started studying kidneys from a sort of physiology point of view. What happened is this, what they needed was some sort of way of measuring how well the kidney was able to handle certain compounds in terms of getting them out of the body (urea and assorted other garbage). What they came up with is a concept referred to as CLEARANCE. The idea was if you have some foreign compound in your body, it’s been eaten or injected in or something like that and it’s traveling, these molecules are traveling through your circulation, your kidney is trying to clear it out. Hence the term “clearance.”

Dr. Joel Schiff – The definition they gave to define clearance is this, the clearance of a compounds called X is, and there are 2 ways to express it, the rate of eliminating X divided by the concentration of X in the plasma. There’s an equivalent version because the rate of eliminating some compound in the urine is equal to the concentration of that compound in the urine times the amount of urine, the rate of urine output. Again divided by the concentration of that compound in the plasma. Now you are going to say that’s kind of weird looking and it seems kind of arbitrary and why would anyone be interested in this formulation? You’re thinking that right now, and who cares? The reason this concept of clearance became very popular and very useful is that it has 2 properties. One is that it generally deals with parameters in this equation that could be measured in the 1880s. Any idiot with a graduated cylinder can measure rate of urine output. By doing the kind of stuff you used to do in your chem labs in college, you could determine the concentration of something in a solution. All you need is to find something that will precipitate it out and you did the weighing and the calculation and so on and so forth after you eliminate the water. So you can get the concentration of something in urine and you can get the concentration of something in plasma and so on. The first thing was it was a practical utility, they could do it. The second reason is this, this is always proportional to the concentration of something in plasma. In other words if you have a lot of something in plasma, then presumably you will be getting rid of it faster, in proportion. If you have little, you will be putting out very little in the urine. In fact, the whole concept of getting rid of stuff being referred to as clearance increased in utility as the century, the next 100 years or so, passed along because people realized that this is specifically referring to urinary clearance or renal clearance. There are other ways your body gets rid of stuff. If you have volatile compounds you might be getting rid of it in breathing. So you can use the term clearance to analyze a particular route of elimination (r-o-u-t-e of elimination) of a compound. By comparing it and having it all relative to the concentration in plasma,

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you can get a measure of the proportional ability of your kidneys, in this case, for renal clearance to handle/to get rid of that particular compound. And then a serendipitous discovery made the whole concept of clearance even more important later on, and I will get to that.

Dr. Joel Schiff – So if you took some, hopefully very harmless, compound into the body/into the blood stream, let’s say. You have an IV drip that drips the stuff in and it maintains a certain plasma concentration and you can draw blood periodically to make sure it’s relatively stable. You do the analysis now and then 20 minutes from now and then 40 minutes from now and so on. And you measure the numerator of the fraction, either the concentration in urine times the rate of urine output is the more useful form generally. You can get an idea of the clearance of the compound. The reason they defined it this way was a totally conceptually wrong way of looking at things because the way they did it was.. the way they thought about it, or at least some of them, was that over a period of time because these are all rates if you notice, over a period of time.. ..alright you have a certain concentration in your plasma of whatever this compound is and you have a certain total volume of plasma. The amount that you got rid of is the amount that was stored/found in a certain volume of plasma, a certain number of milliliters of plasma. So they had this weird concept that by, let’s say if you had a clearance of 500 ml/hour then over the course of an hour your body is getting rid of the amount of this compound that is dissolved in 500 ml of plasma. Which sort of makes NO SENSE at all because it’s not as though you’ve got it distributed throughout your bloodstream except in those 500 mls, which had it cleared out. So it was very confusing and a very wrong way of looking at things. But it’s a useful concept especially when you want to compare the way your body handles certain compounds because you can measure the amount of concentration in the plasma, you draw up some blood sample. You can measure the amount of the concentration in urine and you can certainly measure the rate at which urine is produced. Just to avoid temporary sorts of things, most of these evaluations were done over a 24 hour period. See people would be walking around with gallon jugs.

Cinulin = Rate of elimination

[ Inulin ] plasma=¿

Rateof Filt .[ Inulin ] plasma

Rate of Filt Amt of Inulin filtered = [Inulin]Filtrate x GFRInulinDr. Joel Schiff –What made this suddenly very useful was this, the discovery somewhere along the line of a compound called INULIN, which had some interesting properties. What is inulin? It’s a polymer of fructose isolated from dahlia bulbs (in case anyone cares). But it has the following properties: inulin is freely filtered at the

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INULIN Freely Filtered NOT Reabsorbed No Secretion


glomerulus because it’s not that large a molecule and it’s very water soluble. I mean it’s not big enough to be a starch or anything like that. So it’s freely filtered which means the concentration in the initial glomerular filtrate is the same as the concentration in plasma. Once it gets into the nephron, it’s not reabsorbed and there is no secretion. Like those ads for orange juice, it’s un-fooled around with. What went in at the glomerulus is what comes out in the urine. The amount of the inulin that was filtered at the glomerulus all goes out into the urine. Nothing is added and nothing taken away. Of course they developed a quantitative test to measure the concentration of inulin. What’s the advantage of having such a compound? Well, the amount of inulin that comes out in the urine, if you measure the clearance of inulin, the concentration of inulin in the.. ..or instead let’s look at the left version. The rate of eliminating X, that is the amount of inulin that comes out into the urine per hour, divided by the concentration of inulin in plasma. But the rate of elimination of inulin, because nothing is added and nothing is taken away, is equal to the rate of filtration of inulin in the glomerulus. --That’s not an eraser, what the hell is this? It’s an eyeglass case. Anyone need an eyeglass case? It’s been there for months.-- So that equals rate of filtration at the glomerulus over the concentration in plasma. Because whatever is filtered is what ends up going out in the urine. But the rate of filtration is how much inlulin? Well multiply the concentration of the inulin in the filtrate times the rate at which you are producing filtrate because every mL of filtrate you produce (glomerular filtrate) will have the concentration of inulin times that as the amount of inulin that is filtered.

Cinulin = Rateof Filt .

[ Inulin ] plasma

Rate of Filt Amt of Inulin filtered = [Inulin]Filtrate x GFR

Dr. Joel Schiff –Now, so you go to a person and you measure the rate of elimination. You take a sample of his urine and you can measure how much urine he’s putting out and you can measure the concentration of inulin in his urine. You divide that by the inulin in the plasma to get the clearance of inulin. But remember inulin is freely filtered so the concentration in the filtrate is equal to the concentration in the plasma. So what does that give you? The clearance of inulin is equal to the GFR . So here with the type of equipment that can be found nowadays in a high school chemistry lab, you can measure GFR of a person or an animal. Now, why would you want that? One of the things you want is to see if a person has a clinical issue perhaps, or just as part of a checkup, you want to see if his kidneys working fine. One good test of are the kidneys working fine is can he maintain a steady GFR from year to year to year? It may not be exactly 180, as I say different people have different sizes and so on so it might differ in terms of their GFR. But chances are they will have the SAME GFR now and a month from now and a year from now. If

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Cinulin = GFR


suddenly there’s a change in the GFR, something is going on. Especially when you have suddenly massively endemic conditions, such as diabetes, that have a way of affecting the kidney. So this is a quick and dirty way of measuring GFR. Of course, you have to.. over the time interval in which you collect urine, you have to make sure there’s a reasonably steady concentration of inulin in the blood stream.

Dr. Joel Schiff –So what they do now a days is they don’t measure inulin anymore. There are 2 other roots of making this even more convenient that have to do with our modern understanding of what goes on in the world. One is instead of injecting inulin you infuse.. .. there are a number of iodine-containing compounds where you have a radio labeled isotope of iodine. So instead of having to measure the concentration in the plasma, all you have to do is measure the level of radiation on a blood sample. So that even gets you out of the chemistry lab because you can use the same technique for the same isotope to determine the concentration in the urine.

Creatinine ClearanceDr. Joel Schiff –Second thing you can do is don’t even inject inulin. You find that there are other compounds naturally produced at a fairly steady rate in the body that are also neither reabsorbed nor secreted into the filtrate and have this same inulin-type property. What is mainly done now a days is they measure the clearance of the CREATININE. Think back to when I delivered 4 times, because of this weather we are having, the lecture on muscle in Basic Tissues. There’s a compound creatine that is present in skeletal and cardiac muscle that acts as a phosphate/phosphorylation buffer. If your body is resting and produces lots of a ATP then it transfers the phosphate to creatine to form creatine phosphate, and then it goes the other way when you are doing a lot of work and you need more ATP and so on. Part of the metabolism of creatine, because there’s a constant breakdown and turnover of muscle tissue in your body, is that the liver converts the creatine into creatinine at a fairly steady rate so it’s as though you’re infusing this compound creatinine at a steady rate. Since creatinine is likewise similar to inulin, neither secreted nor reabsorbed in the nephron, what you do is you measure the clearance of creatinine. The absolute number doesn’t really do anything for you. But if there is a change it might indicate there's something going wrong with the kidney. Ok I think I am out of time so we will stop right about here. Is this group Monday or Tuesday? A? That means absolutely nothing to me (laughs). Yeah I know except on Fridays you are early but one group has Monday and one group has Tuesday. So you are Tuesday, ok so we will see you Tuesday and pick it up there.

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36: Renal Physiology IV