Glomerular Filtration Rate (2)

$GFR = ( U_x * U_"vf" )/ P_x$

$"Concentration in Urine"$

$"Urine Flow Rate"$

$"Concentrtation in Plasma"$

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[Renal Physiology] This equation computes the Glomerular Filtration Rate, GFR, the flow rate of filtered fluid passing through the glomerular capillaries of the kidneys. This GFR is equal to the Clearance Rate when a solute is filtered and not reabsorbed or secreted. In the specific case of an amount of substance passing through the kidney which is not reabsorbed, the amount of a substance introduced into the blood, which is then found in a quantity of urine can be used to measure the GFR.

Inputs to this equation are:

$U_x$ - the mass amount per unit volume measured in the urine of the substance introduced (typically in grams/ cubic centimeter or other mass per unit volume unit)
$P_x$ - the mass amount per unit volume measured in the plasma of the substance introduced (typically in grams/ cubic centimeter or other mass per unit volume unit)
$U_"vf"$ - volumetric flow of urine from the kidneys

The average GFR is around 125 ml/min

Notes

DERIVING THE GFR EQUATION

The mass of the substance excreted during the time urine is collected defines the concentration of the substance in the urine, $U_x$ . This concentration in the urine collected can be compared to the plasma concentration of the same substance introduced into the blood and to the volume flow of urine, $U_vf$ . From the ratios of these measurements we can derive the estimate of the volumetric flow of plasma that was filtered, thus the GFR.

USING THE GFR

The GFR is a metric used in characterization of the level of kidney function. The GFR is used to identify stages of kidney disease as shown in the Table: Stages of Kidney Disease. Specifically, the GFR estimates how much glomerular filtrate is formed in all nephrons of both kidneys per minute.

Glomeruli are the tiny filters in the kidneys that filter waste from the blood.

The following table depicts the stages of Kidney Disease, showing the relative values for the Glomerular Filtration Rate associated with each stage. The lower the number the more damage has been done to the kidneys at progressive stages of Chronic Kidney Disease (CKD).

STAGES OF KIDNEY DISEASE

(CKD)	Description	GFR (mL/min)
Normal	Normal kidney function in this range of GFR	Approx 125
1	Normal kidney function but urine findings or structural abnormalities or genetic trait point to kidney disease	More than 90
2	Kidney damage and mild decrease in GFR	60 to 89
3	Moderate decrease in GFR	30 to 59
4	Severe decrease in GFR	15 to 29
5	Kidney failure (dialysis or kidney transplant needed)	Less than 15