Renal clearance

Renal clearance is one of the pharmacokinetic parameters that characterize each drug. This parameter indicates the type of biological mechanism that a given substance undergoes when it arrives in the renal tubule.

What is renal clearance?

Renal clearance is the volume of ultrafiltrate that passes into the kidney and is deprived of a certain amount of drug over a certain unit of time. Mathematically, renal clearance is obtained by applying the following formula:

Where U stands for the urinary concentration of the drug, V stands for the volume of urine excreted per minute and finally P stands for the plasma concentration of the drug.

To understand if a substance undergoes the fate of ultrafiltration, reabsorption or secretion, three specific substances must be used. The substances used are inulin, glucose and p-amino hippuric acid (PAI). After having calculated the reference clearance for each of these substances, a comparison between this and the detected clearance is sufficient; on the basis of the result obtained from the comparison it can then be determined whether the substance can be filtered, reabsorbed or secreted.
Now let's take some examples to better understand the concept of renal clearance.
The first substance examined is inulin, which is only ultrafiltered at the level of the renal glomerulus, so it is neither secreted nor reabsorbed. Inulin is our reference substance for calculating the clearance of a substance that is only ultrafiltered . Its clearance value is 130 ml / min which also corresponds to the glomerular filtration rate.
The second substance examined is glucose, which is completely reabsorbed and its clearance value is zero. Any substance that has a clearance between 0 and 130 is ultrafiltered and reabsorbed, thus exhibiting both mechanisms.
The third and last substance allows us to obtain the clearance of all those substances that are only secreted. The substance under consideration is p-aminoippuric acid (PAI) which is eliminated in the urine only by secretion. Its renal clearance is 650 ml / min.
The three values ​​of the substances examined are:

  • Inulin clearance = 130 ml / min = glomerular filtration;
  • Glucose clearance = 0;
  • PAI clearance = 650 ml / min.

To determine if the substance can undergo one or more elimination mechanisms it is sufficient to make a ratio (R) between the clearance of the substance whose behavior we want to know and the clearance of inulin.

 

R = 1 filtered substance 130/130 = 1; R <1 reabsorbed substance value between 0 and 130/130; R> 1 secreted substance value between 130 and 650/130.

Total clearance

It is the sum of the clearance of all organs responsible for the metabolism and elimination of drugs. The intestine and liver can also eliminate a drug; there is also the enterohepatic circle.

CLearance tot = CL hepatic + CLrenal + CLother (sweat, tears, etc.)

 


Other articles on "Renal clearance"

  1. Renal excretion of a drug
  2. Clearance and biliary excretion
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