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PLASMA CLEARANCE AND RENAL BLOOD FLOW

Lecture-6Dr. Zahoor

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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 timePlasma 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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RENAL BLOOD FLOW

RENAL BLOOD SUPPLY

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Renal blood flow and Oxygen consumption

Oxygen consumption per 100 g of renal tissue is 5 ml/min. which is next highest to the heart (8ml/min).

Total renal blood flow is ≈ 1200/min.

90 % of RBF goes to the renal cortex, 9% goes to outer medulla and 1% to the inner medulla

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BLOOD FLOW THROUGH THE KIDNEYSTotal blood flow through the kidneys can be

calculated from renal plasma flow and Hematocrit (percentage of RBC in the blood)

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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