Renal clearance-a guide of kidney functions

Renal clearance provides essential information about excretory functions: filtration, tubular reabsorption, tubular secretion, and renal blood flow.

1.	Introduction
2.	How to determine
3.	Calculation
4.	Average value
5.	Utility

The renal clearance value of a given substance is the volume of plasma in milliliter wholly cleared of that given substance in one minute by the kidneys;  it is expressed as ml per minute( ml/minute).

Clearance compares the amount of substance removed from the plasma per unit time for permanent with the amount of substance excreted in the urine per minute. Plasma concentration rather than whole blood consideration is used to calculate renal clearance.

Normal plasma flow is 700 ml per minute. We know that plasma is 55% in the blood and form elements of blood are 45%. So when plasma flow is 55ml per minute, total renal blood is 100 ml per minute.  Then the total renal blood flow is 100 x 700 / 55, approximately 1273 ml per minute.

Calculation  of renal clearance of a given substance=

 Urine flow per minute X urinary concentration of the substance/: arterial plasma concentration.

 Renal excretion rate  =Urine flow per minute X urinary concentration of the substance.

Renal clearance of a given substance= Renal excretion rate of the substance its arterial plasma concentration.

Calculation:

Suppose the substance is X, then the calculation is

The renal clearance rate of X=Urine flow per minute of X/plasma concentration of X.

 If a substance is not metabolized in the tissues, the arterial level equals the plasma level of that substance.

  The urea clearance (rate in ml per minute):

 Urinary concentration of urea into urine flow per minute/plasma concentration of urea.

Average value

 The average value of  Urea is 20 mg per ml of urine.

 The urine flow rate is 1 ml per minute.

Renal excretion rate of urea=20×1=20.

 The plasma concentration of urea is 30 mg per dL.

 Urea clearance = 20 X 100 /30  = 67 ml per minute

 Utility of clearance test:

Renal clearance provides essential information about excretory functions: filtration, tubular reabsorption, tubular secretion, and renal blood flow.

 1. Determination of glomerular filtration rate.

 2. Estimation of renal plasma flow and renal blood flow.

 3. Measurement of tubular filtration rate.

 4. In the calculation of tubular secretory capacity.

  5. In quantifying osmotic and free water clearance

  6. Assessment of excretion of waste products.

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