Renal clearance

Definitions & Key takeaways

Renal clearance is the rate at which certain substances are removed from the plasma by the kidneys. It helps to measure how well the kidneys are filtering waste products from the blood. A higher renal clearance suggests the substance may be cleared almost completely in one pass through the kidneys, whereas a low value suggests that a substance may not be eliminated by the kidneys at all. In calculations, the renal clearance (C) of a substance x �, will be directly proportional to the urine concentration of the substance x ([U]x) and the urine flow rate (V), and inversely proportional to x's plasma concentration ([P]x): C=[U]x V[P]x

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Introduction0:00–0:53

In order for the body to function properly, it needs a way to get rid of toxins and other waste materials. That's where the kidneys come in.
Their main function is to filter the blood and remove any unwanted substances from the body. Now, the first step in blood filtration happens at the glomerulus, a tiny bed of capillaries surrounded by the Bowman's capsule.
The glomerular filtration barrier is made up of three layers and together they separate the blood inside the glomerular capillaries from the flu inside the Bowman's capsule.
They work like a sieve allowing water and some solutes in the plasma like sodium to pass into Bowman's space while keeping negatively charged particles like proteins or large particles like red blood cells in the blood.
The filtered fluid now called preurine leaves the Bowman space and travels through the nephron. The nephron is the basic unit of the kidney and it is essentially one long tube bent into au shape, different sections of this tube, either reabsorb substances back into systemic circulation or actively secrete them into the Nephron to be excreted in urine, renal clearance of a substance refers to how quickly a particular substance is removed from the plasma by the kidney and excreted in urine.

Nephron0:53–1:11

Renal clearance1:11–1:30

So something with a high renal clearance means that it will be quickly removed from the blood and vice versa. There's a formula to calculate renal clearance for some substance.
Let's call it X. In this formula, C stands for the renal clearance, which is the volume of blood plasma that's cleared of the substance over time and minutes.

Clearance formula1:30–3:19

C equals the concentration of the substance in urine. U multiplied by the urine flow rate.
V dot which is the amount of urine excreted over time in minutes. All of that's divided by the plasma concentration of the substance P.
So if the urine concentration is high, but the plasma concentration is low, then that must mean that a lot of substance was removed from the blood leading to a high renal clearance.
As a general rule, small uncharged substances like inulin, which is a small inner polysaccharide molecule have a relatively easy time passing through the glomerulus.
As an example, let's say that in a 24 hour period, a man has 2 L of urine and that his plasma sodium concentration is 145 mill equivalents per liter.
Whereas his urine sodium concentration is 190 mi equivalence per liter. Using this information, let's calculate his renal clearance for sodium.
First, we need to calculate his urine flow rate, which is the urine volume divided by time. So that's 2000 mL divided by 1440 minutes, which equals 1.39 mL per minute.
Since urine concentration is 190 milli equivalents per liter. We multiply that by 1.39 mL per minute and divide by the plasma concentration which is 145 milli equivalent per liter.
This equals 1.82 mL per minute. So that means that 1.82 mL of plasma is cleared of sodium per minute.
So we know how much plasma is cleared of sodium per minute by the kidneys. But we don't know if any of the sodium is being reabsorbed or secreted into urine by the nephrons.

Inulin3:19–4:28

This is because clearance is the sum of all of the reabsorption and secretion that occurs for a substance. And in order to tease out exactly how much reabsorption and secretion is occurring, we need to compare it to inulin inulin is a polysaccharide that is produced by plants.
It is the one substance that's freely filtered and not actively secreted or reabsorbed. We know this because the filtered fraction, which is how much fluid is reaching the kidneys and passes into the renal tubules is the same for inulin as it is for plasma.
So we can use it to get an accurate estimation of how much fluid is filtered from the renal glomerular capillaries into the filtrate, also known as the glomerular filtration rate or GFR.
When we compare the clearance of substance X to inulin. We get something called the clearance ratio.
This can be calculated simply as the clearance of substance X divided by the clearance of inulin. If the ratio is equal to one, then substance X is the same as inulin and must also be freely filtered and not actively secreted.

Clearance ratio4:28–6:33

If the ratio is greater than one, the clearance of substance X must be greater than inulin. And that means that it must be freely filtered and also actively secreted by the kidneys.
If the ratio is less than one, the clearance of substance X must be less than inulin. And that means that it must not be freely filtered like with albumin or it may be freely filtered.
But then some of it may be getting reabsorbed by the kidney like with glucose. So let's continue with that earlier scenario and assume that we gave that patient an infusion of inulin over two hours.
Let's suppose the urine concentration of inulin is 140 mg per milliliter. And that the plasma concentration of inulin is 1 mg per milliliter.
We can use the same urine flow rate as before which was 1.39 mil per minute. Since urine concentration is 140 mg per milliliter.
We multiply that by 1.39 mL per minute and divide by the plasma concentration, which is 1 mg per milliliter. This equals 194.6 mL per minute.
So that means that 194.6 mL of plasma is cleared of inulin per minute. Now, we can calculate the clearance ratio for sodium, which is 1.82 mL per minute, divided by 194.6 mL per minute, which equals 0.009 0.009 is far less than one.
So that tells us that very little sodium is excreted in the urine. Interestingly, since sodium is freely filtered, it must be extensively reabsorbed by the Nephron to have such a low clearance.
This clearance ratio means that less than 1% of filtered sodium is excreted in the urine. Whereas the other 99% is reabsorbed.

Free water clearance6:33–9:32

Now that we know how to find the clearance rate of a solute. Let's look at the clearance rate of just water called free water clearance.
This tells us if water is being reabsorbed or secreted by the kidneys. Basically, anything that's happening after water is filtered into the bone and space, that's helpful because it tells us if there's a problem with the distal convoluted tubule and collecting ducts which have protein channels called aquaporin that only reabsorb water.
These channels are activated by anti diuretic hormone which is secreted from the pituitary gland. So, free water clearance C H2O is calculated by this equation.
So free water clearance is the urine flow rate. V minus the osmolar clearance co sm and conversely, urine flow rate is the free water clearance plus the osmolar clearance.
We can find cosm by multiplying urine osmolarity usm with the urine flow rate. V and dividing by plasma osmolarity.
POS M This is similar to the equation we use to find the clearance of a solute except now we include all the solutes. Let's work through a simple problem to make this concrete.
Let's say that a woman has a urine flow rate of 1.5 mL per minute. A urine osmolarity of 130 mismos per liter and a plasma osmolarity of 280 mill osmoles per liter.
What would be her free water clearance plugging in the numbers? We get that.
The urine flow rate is 1.5 mL per minute. And the CSM can be calculated by multiplying 130 mi osmoles per liter by 1.5 mL per minute and divided by 280 mill osmoles per liter, which gives us 0.7 mL per minute, which means 0.7 milliliters of plasma is cleared of all the solutes found in the urine every minute by the kidneys.
To calculate the free water clearance, we take 1.5 mL per minute minus 0.7 mL per minute, which equals 0.8 mL per minute, which means 0.8 mL of plasma is cleared of solute free water per minute.
If the free water clearance value is positive, that means that free water is being secreted and the urine is hypo osmolar compared to plasma, meaning the urine is less concentrated.
This could happen if we drink a lot of water and our body wants to get rid of some of it or if the kidneys are not responding to ADH if the value is negative, this means that free water is being reabsorbed and the urine is hyperosmolar compared to plasma, meaning the urine is more concentrated.
This could happen if we're dehydrated or if we're releasing an inappropriate amount of ADH. All right, as a quick recap renal clearance is the rate at which certain substances are removed from the plasma by the kidneys.

Review9:32–10:04

By comparing this rate to the clearance rate of inulin which is freely filtered and not secreted, we can get an idea of if the substance is secreted, reabsorbed or poorly filtered free water or pure water clearance can be similarly calculated by getting the rate of urine flow and subtracting the rate of solute clearance to get the rate that pure water is filtered out of the plasma and excreted by the kidney.