# Renal Clearance. Clearance 20 4444444444 40 12345678910

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17-Jan-2016

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• Renal Clearance

• Clearance40

• Was any of the carriages completely cleared? NoBut can we say that it is as if some carriages were completely cleared? YesIf so, how many carriages were completely cleared?2

• 40Clearance of the train is 2 carriages / stationRemember It is not real (it is only as if) It is related to the train (not the number of passengers)

• Clearance is - Number of carriages of the train cleared of passengers

The minimum number of carriages needed to supply the number of passengers getting off at the stationOR The number of carriages that contained the number of passengers getting off the train

• Calculation of clearance - number of passengers going out of the station number of passengers in each carriage

• Apply to plasma and kidney Kidney Solute X excretedPlasma comes into kidney withmore solute XPlasma goes out of the kidney with less solute X

• Apply to plasma and kidney KidneyExcretion of the solute XPlasma comes into kidney withmore solute XPlasma goes out of the kidney with less solute XClearance of the solute X =Amount of solute X excretedConcentration of solute X in plasmaRenal clearance is taken as PER MINUTE

• Renal Clearance of a substance is - Volume of plasma cleared of the substance by the kidney in one minute

The minimum volume of plasma needed to supply the amount of substance excreted by the kidney in one minuteOR The volume of plasma that contained the amount of substance excreted by the kidney in one minute

• Renal clearance of a substance

Total substance excreted in 1 minuteConcentration of substance in plasma

Conc. of substance in urine x Vol. of urine excreted in 1 minConcentration of substance in plasmaU x V PU concentration of the substance in urine

V volume of urine produced in 1 minute

P concentration of the substance in plasma

• Unit of renal clearance

U x V P

U concentration of the substance in urine mmol/ml

V volume of urine produced in 1 minute ml/min

P concentration of the substance in plasma mmol/ml

mmol/ml x ml/min mmol/min mmol/ml mmol/ml

Unit of renal clearance is ml/min= ml/min=Warning: when performing the calculation make sure that units of U, V and P are similar!

• Renal Clearance of a substance - determination

U x V P

U concentration of the substance in urine collect urine and measure the concentration

V volume of urine produced in 1 minute collect urine for 24 hours and calculate urine output per minute; more accurate as the output varies throughout the day

P concentration of the substance in plasma collect blood during those 24 hours and measure the concentration

• Significance of clearanceMeasurement of clearance can be used to determine

Glomerular filtration rate thereby assessing glomerular function

Renal plasma/blood flow not routinely performed

• Significance of clearanceGlomerular filtration rate the clearance of a substance freely filtered at the glomerulus but not reabsorbed or secreted in the tubules is the value of GFR

Renal plasma/blood flow the clearance of a substance filtered at the glomerulus and completely secreted by the tubules is the same as renal plasma flow

• Significance of clearanceConsider a substance handled by the kidney in the following manner -

Freely filtered at glomerulus

Completely secreted by tubules

• Significance of clearance1. Freely filtered at glomerulus

2. Completely secreted by tubules

The amount of substance excreted comes from the entire plasma flowing through the kidneyCalculate the clearance of the substanceU x V P= total plasma flow through the kidney

• Significance of PAH clearancePara amino hippuric acid (PAH) has these characteristics (at low concentrations)Clearance of PAH = Renal plasma flowTotal plasma flow? No, some blood may bypass the nephrons Clearance of PAH = Effective renal plasma flowTotal renal plasma flow is about 5 -10% more than effective renal plasma flow

• Significance of PAH clearanceIf plasma flow is known, blood flow can be calculated

For this haematocrit needs to be knownclearance of PAH = Effective Renal blood flowFor example: if PAH clearance is 600ml/min, and haematocrit 40%,

600 / (1 - 40%)Effective Renal blood flow ==600 x 100 601 - haematocrit

• Significance of clearanceA substance handled by the kidney in the following manner-

Freely filtered at glomerulus

Not absorbed or secreted by tubules

The amount of substance excreted comes entirely from the glomerular filtrate The amount of substance excreted in 1 minute will come from the glomerular filtrate formed in 1 minute (GFR)

• Significance of clearanceA substance handled by the kidney in the following manner-

Clearance of the substance when calculated will give the volume of plasma that contains the amount of substance excreted in 1 minute

Freely filtered at glomerulusNot absorbed or secreted by tubulesThe amount of substance excreted comes entirely from the glomerular filtrate

• Significance of clearanceThe substance excreted comes entirely from the glomerular filtrateComes from the glomerular filtrate in 1 minute (GFR)=the amount of substance excreted in 1 minutecomes from volume of plasma cleared of the substance in 1 minute (Renal clearance)=

• Clearance as a measure of GFRThe clearance of a substance freely filtered at the glomerulus and not reabsorbed or secreted will be the same as the GFR

• Measurement of GFRProperties of an ideal substance whose clearance mat be determined to measure GFR Freely filtered at the glomerulusNot reabsorbed by the tubulesNot secreted by the tubulesNot metabolised by the nephronNot toxic to the bodyDoes not affect renal function, particularly glomerular functionEasily measured in the plasma and urineFound in the body

• Clearance as a measure of GFRThe clearance of a substance freely filtered at the glomerulus and not reabsorbed or secreted will be the same as the GFR

The best example of such a substance is INULINInulin clearance, therefore, is the same value as GFR

• Measurement of GFRInulin as an ideal substance for measuring GFR

Freely filtered at the glomerulusNot reabsorbed by the tubulesNot secreted by the tubulesNot metabolised by the nephronNot toxic to the bodyDoes not affect renal function, particularly glomerular functionEasily measured in the plasma and urineFound in the body

• Measurement of GFRInulin as an ideal substance for measuring GFR

Found in the body

As inulin is not found in the body it needs to be infused Associated with risks and restrictions for the person

• Alternate substances for measurement of GFRCreatinine for measuring GFR (source of creatinine : muscles)

Found in the body

Has a major advantage over inulin as no infusion is necessaryHowever Creatinine is secreted in small amounts tends to overestimate GFRThe method of measuring serum creatinine gives a falsely higher value (slight) tends to underestimate GFR

• Creatinine clearanceCommonly performed clearance estimationA good indicator of glomerular function

Parameters measured Serum creatinine concentration24-hour urine volumeUrine creatinine concentration

• Clearance ratioIf the clearance of a substance is :

Less than creatinine clearanceReabsorption occurs? Secretion

More than creatinine clearanceSecretion occurs? Reabsorption

• Serum creatinineCould serum creatinine be a substitute for creatinine clearance?

Measuring serum creatinine is much easier than determining creatinine clearance

Reasoning may be ? if GFR decreases creatinine excretion decreases? then serum creatinine level increases? Increasing serum creatinine will indicate decreasing GFR

Is this really so?

• GFR and plasma creatinineWhen GFR falls, plasma creatinine increases but not proportionately the relationship is not linearWhen GFR falls from normal levelsAt lower levels of GFR even small decreases in GFR cause large increases in plasma creatinineThe plasma creatinine increases slowly initially

• GFR and plasma creatinineA high serum creatinine suggests a low GFR

A normal serum creatinine is not proof of normal GFRSerum creatinine is also dependent on a persons muscle mass A high serum creatinine suggests a low GFR because there are no other significant factors that change the serum creatinine

• Renal ClearanceClearance is a concept not a real valueIt refers to plasma - unit : ml/minIt can be determined for any substance excreted in the urineParameters needed for calculation are plasma concentration, urine concentration and urine flow rateThe clearance of certain substances are determined as indicators of renal functionBlood levels of some substances excreted in urine are sometimes used as indicators of glomerular functionBut blood levels do not change proportionately with changes in GFR