Download plasma clearance

Dr. Rida Shabbir
DPT IPMR KMU
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Objectives
 Describe the concept of renal plasma clearance
 Use the formula for measuring renal clearance
 Use clearance principles for inulin, creatinine etc. for
determination of GFR
 Use PAH clearance for measuring renal plasma flow
 Calculate Renal Blood Flow
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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:
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
secreted
 If 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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CLEARANCE
 Inulin is used to measure GFR
 Inulin 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
GFR = Cinulin =
Uin x V
Pin
 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
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CREATININE CLEARANCE
 In clinical practice endogenous substance Creatinine is
often used to find GFR
 Creatinine is end product of muscle metabolism
 Creatinine 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
between
GFR 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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Use of Clearance to Estimate
Renal Plasma Flow
Theoretically, if a substance is completely cleared from
the plasma, its clearance rate would equal renal plasma
flow.
Cx = renal plasma flow
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PAH clearance example
PAH CLEARANCE
 PAH is 90% cleared from the plasma called extraction
ratio
 Renal plasma flow (RPF) can be calculated by PAH
CLEARNCE
UPAH x V
CPAH =
U = 5.85mg/ml
V = 1 ml/min
P = 0.01mg/ml
Therefore, C = (5.85 × 1)/0.01
RPF = 585 ml/min
PPAH
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PAH CLEARANCE
 Actual Renal Plasma Flow can be calculated by
dividing 585/0.9 = 650ml/min
 We 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
KIDNEYS
 We apply formula
RBF =
RPF
1 - Hct
If renal plasma flow = 650ml/min
HCT = 0.45
Therefore,
650
RBF =
RBF =
1 - 0.45
650
0.55
= 1182ml/min
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REABSORBTION AND SECRETION
OF SUBSTANCES
 If 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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Inulin
Glucose
Plasma Clearance
For substances
handled in
different ways by
the kidney
Urea
PAH
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CLEARANCE RATE







Substance
Glucose
Na+
K+
Inulin
Creatinine
Urea
PAH
Clearance Rate (ml/min)
0
0.9
12
125
140
70
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) GFR
ii). 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). Glucose
ii). Inulin
iii). Creatinine
iv). Urea
v). PAH
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Thank you
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