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RENAL PLASMA CLEARANCE It is the volume of plasma cleared of a substance per unit time Useful way of quantifying the excretory function of the kidneys the rate at which blood flows through the kidneys the basic functions of the kidneys: glomerular filtration, tubular reabsorption, and tubular secretion. Plasma clearance is calculated by the formula: C s = Us x V Ps C s = clearance of a substance U s = urine concentration of a substance V = volume of urine ml/min P s = plasma concentration of a substance 3
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PLASMA CLEARANCE
Dr. Rida ShabbirDPT IPMR KMU
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ObjectivesDescribe the concept of renal plasma
clearanceUse the formula for measuring renal
clearanceUse clearance principles for inulin, creatinine
etc. for determination of GFRUse PAH clearance for measuring renal
plasma flowCalculate Renal Blood Flow
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RENAL PLASMA CLEARANCEIt is the volume of plasma cleared of a substance per unit timeUseful way of quantifying
the excretory function of the kidneys the rate at which blood flows through the kidneysthe basic functions of the kidneys: glomerular filtration, tubular
reabsorption, and tubular secretion.Plasma clearance is calculated by the formula: Cs = Us x V Ps
Cs = clearance of a substance Us = urine concentration of a substance V = volume of urine ml/min Ps = plasma concentration of a substance
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CLEARANCE Inulin clearance is used to measure GFR
{Glomerular Filtration Rate} WHY ?Because Inulin is filtered but not reabsorbed
or secretedIf a substance X [Inulin] is filtered from
125ml/min, then it is excreted but not reabsorbed, not secreted, therefore, 125ml of filtrate is cleared of substance X .
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CLEARANCEInulin is used to measure GFRInulin is a carbohydrate, polysaccharide, not
produced in the body but found in certain plants
Inulin is infused I/V continuously to maintain a constant plasma level
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CLEARANCE
If Uin = 30mg/ml of urine, V = 1.25ml/min, Pin = 0.3mg/ml in plasma then C Inulin = (30 × 1.25)/ 0.3 GFR = 125ml/min
Cinulin = Pin
Uin x VGFR =
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CREATININE CLEARANCEIn clinical practice endogenous substance
Creatinine is often used to find GFRCreatinine is end product of muscle metabolismCreatinine is filtered, not reabsorbed ,but
slightly secreted, therefore, creatinine clearance is not so accurate for measurement of GFR as Inulin
But creatinine clearance provides close approximate value and easy to use as it does not require I/V injection
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Steady-state relationship
betweenGFR and serum
creatinine concentration
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PARA AMINO HIPPURIC ACID (PAH)PAH clearance is used to measure renal
plasma flow WHY ? Because PAH is filtered, not reabsorbed but
secreted from all the plasma that flows through the kidneys
Renal plasma flow is 650ml/min
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Theoretically, if a substance is completely cleared from the plasma, its clearance rate would equal renal plasma flow.
Use of Clearance to Estimate Renal Plasma Flow
Cx = renal plasma flow
PAH clearance example
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PAH CLEARANCEPAH is 90% cleared from the plasma called
extraction ratio Renal plasma flow (RPF) can be calculated by
PAH CLEARNCE U = 5.85mg/ml V = 1 ml/min P = 0.01mg/mlTherefore, C = (5.85 × 1)/0.01 RPF = 585 ml/min
CPAH = UPAH x VPPAH
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PAH CLEARANCEActual Renal Plasma Flow can be calculated
by dividing 585/0.9 = 650ml/minWe are dividing by 0.9 because PAH removed
from the plasma i.e extraction ratio is about 90% in normal kidneys
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BLOOD FLOW THROUGH THE KIDNEYSWe apply formula If renal plasma flow = 650ml/min HCT = 0.45Therefore,
RBF = RPF1 - Hct
RBF = 6501 - 0.45
RBF = 6500.55
= 1182ml/min
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REABSORBTION AND SECRETION OF SUBSTANCESIf a substance has clearance rate equal to
Inulin then substance is only filtered but not reabsorbed, not secreted
If a substance clearance rate is less than Inulin clearance, then substance has been reabsorbed by nephron tubule E.g. Urea
If clearance rate of a substance is greater than that of Inulin then substance must be secreted by nephron tubule e.g. creatinine
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Plasma Clearance For substances handled in different ways by the kidney
Inulin
Urea
Glucose
PAH
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CLEARANCE RATE Substance
GlucoseNa+
K+ InulinCreatinineUreaPAH
Clearance Rate (ml/min)
00.912
12514070
600
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Clearance Discussion
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Q1. What is Plasma Clearance?
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Q2. Which substance is used to measure i) GFRii). Renal Plasma Flow
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Q3. What is the formula for the clearance?
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Q4. If a substance is filtered, then some of it is reabsorbed, its clearance will be less or more than Inulin clearance?
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Q5. If a substance is filtered, then secreted, its clearance will be less or more than Inulin clearance?
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Q6. What is the plasma clearance of i). Glucoseii). Inuliniii). Creatinine iv). Ureav). PAH
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Thank you