Renal Clearance Equations

by Thad Wilson, PhD

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    00:01 So what exactly is clearance? Clearance is the ability of the kidney to filter the blood.

    00:09 Now in this filtering process, although we can think of it as a filter that you might use for drinking water or even some sort of sieve type of filter.

    00:20 The body's filtering system works in a slightly different way.

    00:24 The first thing thus work in this manner in which it will have smaller pore sizes to only let certain substances through.

    00:33 However, there are a lot of small substances that we will need to retain, and therefore we'll have specific transporters in place to reabsorb those after we've already cleared them.

    00:46 So again, what is clearance? As blood travels through the Bowman's capsule, a portion of that blood will be filtered.

    00:59 And that portion that is filtered, it is cleared, and that is clearance.

    01:07 Now, know that once you have certain substances cleared, you could still reabsorb them.

    01:13 But clearance is that initial filtering of the blood.

    01:19 We have a few other relationships that we need to bring out.

    01:24 And that is, there are a number of factors that we need to try to calculate.

    01:30 And we can use various calculations in the renal system by using this clearance equation.

    01:37 Now, the clearance equation is looking at the concentration of a substance in the urine times the urine formation rate, which is V, over the plasma concentration of the substance.

    01:49 And that clearance equation won’t change throughout our various substances that we’re going to look at.

    01:58 Interestingly though, you can pick a certain substance to look at that will give you insight into the function of the kidney.

    02:05 Inulin is one of these substances. We can use inulin as an index to measure glomerular filtration rate.

    02:14 And how you do that is take glomerular filtration rate times the concentration of inulin in the urine, times the urine formation rate, divided by the plasma concentration of inulin.

    02:28 Creatinine is another way to measure glomerular filtration rate. We utilize the very same equation.

    02:36 The only thing we’re doing is substituting creatinine for the X.

    02:40 So here, we have the urine concentration of creatinine, times the urine formation rate, divided by the plasma concentration of creatinine.

    02:50 So both of these two substances can be utilized to measure glomerular filtration rate.

    02:56 Inulin is a little bit more accurate, but creatinine is made in the body, and therefore, it’s more functional.

    03:03 So you can measure someone’s creatinine much easier than you can induce or introduce inulin into someone.

    03:12 The last substance that we can utilize in this kind of clearance equation is PAH.

    03:18 PAH is utilized to help us for renal plasma flow. We use the same equation. All we’re doing is substituting PAH for the X.

    03:29 So here, we have renal blood flow, or renal plasma flow, equals the urine formation concentration of PAH, times the urine formation rate, divided by the plasma concentration of PAH.

    03:45 So let’s now walk through these in a cartoon format in case you’re one of those individuals that struggle a little bit with equations. So let’s go through this in our picture form here.

    03:59 So what happens to inulin? Inulin, as it travels through the afferent arteriole into the glomerular capillary, is what we call freely filtered.

    04:10 And what freely filtered means is it travels through that filtration barrier freely.

    04:16 The filtration barrier doesn’t block it or say hold back. It allows it to travel through.

    04:24 Interestingly, inulin is not reabsorbed. There are no specific transporters for inulin. Inulin is also not secreted.

    04:36 So what this does is, everything of only substances of inulin that are filtered are ended up excreted out into the urine.

    04:46 There’s no addition or subtraction to that volume.

    04:53 Creatinine looks very similar. It is also freely filtered. It is not reabsorbed.

    05:00 However, there is a tiny amount that is secreted. So in this case, that part is different than inulin.

    05:08 Interestingly though, since you know about how many transporters are there, people oftentimes use a fudge factor for that small amount of secretion.

    05:18 That’s why you can still use creatinine as a marker for glomerular filtration rate.

    05:24 It is just a little less accurate because the secretion rate is not fully accounted for.

    05:34 Properties of PAH. It is also freely filtered. Interestingly, it’s still not reabsorbed.

    05:42 And finally, it is secreted fully, meaning that it is going to secrete as much, or based upon a transport maximum, across from the blood through the renal tubule cells into the tubule fluid.

    05:56 So in this case, you have both the freely filtered amount and the secreted amount that are measured in the urine.

    06:08 The final components that we need to do is convert renal plasma flow into renal blood flow.

    06:14 And how you do that is by taking into account the hematocrit.

    06:19 Remember from the cardiovascular section that hematocrit is the amount of red blood cells in the blood.

    06:26 So with this, you take the renal plasma flow and you divide it by 1 minus the hematocrit, and that will yield renal blood flow.

    06:35 Other important equations that we have in the kidney in terms of the system is the filtered load of a substance.

    06:44 And what we mean by a filtered load? Because this is kind of hard to think about, is the glomerular filtration rate multiplied by the plasma concentration of a substance.

    06:55 So if you take something like sodium, if you know the plasma concentration of sodium, you know the glomerular filtration rate, you can figure out how much in terms of an absolute amount a load that was delivered to the Bowman’s space.

    07:12 The last item that’s important in renal physiology is the filtration fraction.

    07:18 The filtration fraction is the glomerular filtration rate divided by the renal plasma flow.

    07:24 So what this is in practical terms is the amount of flow going in the afferent arteriole through the Bowman’s capillaries, and then out the efferent arteriole, how much of that flow was filtered.

    07:39 So that is glomerular filtration rate divided by renal plasma flow.

    07:45 I know we went through a lot of different equations, but these are important renal functions.

    07:51 And if you understand these kinds of equations, you’ll be able to understand the concept that is behind them.

    About the Lecture

    The lecture Renal Clearance Equations by Thad Wilson, PhD is from the course Renal Physiology.

    Included Quiz Questions

    1. Para-aminohippurate
    2. Inulin
    3. Creatinine
    4. Carnitine
    1. Inulin
    2. Hippuric acid
    3. Carnitine
    4. Insulin
    1. GFR and plasma concentration of sodium
    2. Plasma concentration of sodium
    3. Plasma concentration of sodium and renal blood flow
    4. GFR and renal plasma flow
    5. Plasma concentration of sodium and renal plasma flow
    1. A small amount is secreted by the kidneys.
    2. It is not freely filtered.
    3. It is partially reabsorbed in the kidneys.
    4. It is not secreted by the kidneys.
    5. It is fully reabsorbed in the tubules.

    Author of lecture Renal Clearance Equations

     Thad Wilson, PhD

    Thad Wilson, PhD

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    Super thorough
    By Nimrah A. on 29. January 2018 for Renal Clearance Equations

    Brief and to the point with the material. The visual was also quite helpful