Glomerular Filtration Rate 

The amount of filtrate formed in the capsule per minute is the glomerular filtration rate (GFR). On the average the GFR is around 125ml/min. As the GFR is the primary determinant of urine volume and thus of the excretory potential of the kidneys, it is medically important to be able to measure this quantity. Measurement of GFR can be done by utilising the Clearance principle.
Clearance is defined as the volume of plasma cleared of a given substance per unit time. The formula to calculate the clearance of a substance is given as:
C_{x} = 
(U_{x}) x (V) 
P_{x} 
Where C_{x} is the clearance of substance x, U_{x} is the urine concentration of substance x, V is the urine flow rate and P_{x} is the arterial plasma concentration of substance x. Thus the units of clearance are volume/unit time (usually ml/min).

