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Renal Clearance

Kidney Clearing Floor with Broom
tantheman
Renal clearance is a clinical index used to assess kidney function. It is expressed by the equation Clearance of X = (Urine Concentration of X) x (Urine Flow Rate) / (Plasma Concentration of X) or Clearance = (Ux x Vx) / Px. Conceptually, it describes the volume of plasma that is cleared of a given substance per unit time. Clinically, this is important when estimating the glomerular filtration rate or GFR, which specifically uses the renal clearance of creatinine to provide an objective measure of kidney function. Creatinine specifically is used because it is freely filtered into Bowman's Capsule and is neither reabsorbed nor secreted actively. This means that the amount of creatinine filtered out of the plasma into the urine is entirely dependent on the volume that the renal nephrons are able to handle and filter.
6 KEY FACTS
CHARACTERISTICS
Estimates Volume of Substance Cleared by Kidney Per Unit Time
Kidney with Measuring-cup and Stopwatch

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.

Creatinine Clearance Used to Estimate GFR
GFR-Gopher with CREAm

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.

EQUATION
Cx = (Ux/Px) * V
Kidney Clearing, Concentrated Urine Drop, Plasma-TV Concentrate, Urinal with Clock

Renal clearance (Cx) is equal to the urine concentration of substance X (Ux) divided by the plasma concentration (Px) multiplied by the urine flow rate (V).

VARIABLES
Urine Concentration of Substance X (Ux)
Concentrated-urine in U-shape

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.

Plasma Concentration of Substance X (Px)
Plasma-TV Concentrate

The plasma concentration of substance X (Px) is typically obtained from a blood draw.

Urine Flow Rate (V)
Urinal with Clock

The urine flow rate (V) 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 24-hour period and measured.

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