Renal clearance tells us what volume of plasma is completely cleared of a substance in a given amount of time. For example, if I have one liter of plasma with four particles of substance and I remove all four particles in an hour, my renal clearance is one liter per hour. If however I clear only two particles out of four in a given hour, my renal clearance is half a liter per hour.
One specific clinical use of calculating renal clearance is in calculating the clearance of creatinine, which is used to estimate glomerular filtration rate, or GFR. Creatinine specifically is used because it is freely filtered at the glomerulus and is neither actively absorbed or excreted. In other words, the urine concentration of creatinine compared to the plasma concentration is dependent only on how much volume is able to be cleared by the kidney, which is dependent on its function and ability to filter. In comparison, the concentration of other substances, for example sodium, in the urine versus plasma is dependent on other factors such as endocrine signaling and volume status, which may lead to active excretion or absorption of sodium independent of renal function.
The urine concentration of a substance (Ux) expressed in units of concentration such as mg/dL or mmol/L must be measured in order to calculate renal clearance of a substance.
The urine flow rate is expressed as volume per unit time, and reflects the volume of urine produced in a given amount of time. When trying to measure this clinically, urine is typically collected over a twenty-four hour period and measured.
Finally, the product of urine flow rate and urine concentration of substance X is divided by the plasma concentration of substance X, which is typically obtained from a blood draw. You will notice that the units of concentration from dividing urine by plasma concentration will cancel out, leaving units of volume per unit time.
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