Clearance Flashcards
How is renal function quantified?
Clearance
GFR
RPF
Fractional excretion
What is renal clearance used to measure?
GFR, RPF and determine renal handling of different substances
What is renal clearance?
The volume of plasma cleared of a substance is by kidneys per unit time
How much urea is excreted per min?
Urea excretion= 6 mg/ml x 2ml/min=12 mg/min
What is the clearance of urea?
Urea clearance= 12mg/min/0.2nmg/ml= 60 ml/min
For which substances can renal clearance be measured?
Anything and everything that gets fired!
The value obtained will depend on how the substance is handled by kidneys:
-If the substance is not excreted in the urine, it’s clearance = zero -Ex. (Glucose)
- If the substance is filtered and not reabsorbed or secreted in the urine, it’s clearance= GFR
- If the substance undergoes net Reabsorption, it’s clearance will be less than that of Inulin- Ex. (Na+, urea, CL-)
- If the substance undergoes net secretion, it’s clearance will be greater than that if Inulin
- If the substance undergoes net secretion, it’s clearance will be greater than that of inulin-Ex. PAH
What is the filtered load?
Excreted amount (Inulin )/(mannitol)
How do we know if it’s net secretion or net Reabsorption?
There is net Reabsorption of a substance if excretion is < filtered load
There is net secretion of substance if excretion > filtered load
How can we use clearance to measure GFR?
Need a substance(a) with special properties
- Freely filtered (concentration in Bowman’s space= concentration in plasma)
- Not reabsorbed
- Not secreted
Amount 9f a excreted per minute= amount of a filtered per minute
C1 x V1= C2 x V2
Urine concentration of (a) x urine flow rate= Glomerular filtrate concentration of ( a) x GFR
GFa= Pa
GFR= Ua x V/GFa
What is Inulin?
- polysaccharide of fructose
- molecular weight about 5000 daltons
- molecular diameter about 3nm
What is the clearance of Inulin?
Inulin clearance= GFR Typical value for GFR is about 120 ml/min -freely filtered -not reabsorbed -not secreted
Inulin is cleared from plasma by filtration alone
Since none of filtered Inulin returns to the plasma, volume of plasma cleared of Inulin per min must equal the volume of plasma filtered per min(=GFR)
About 180 liters/day
What are the problems with measuring Cinulin clinically?
- Must be infused intravaneously
- Continual blood sampling required
- Chemical analysis cumbersome
What is a useful alternative to Inulin measure clearance clinically?
Creatinine (not creatine)
- Endogenous (by-product of muscle metabolism)
- Released into blood at relatively constant rate (plasma concentration is fairly stable; therefore only need one blood sample)
- Freely filtered
- Not reabsorbed
- Is secreted in small amounts
True Ccr overestimates GFR by 10%-20%
What is the use of creatinine?
Plasma & urine creatinine concentration
Ccr= Ucr x V/Pcr
What are the 2 drawbacks of creatinine?
- It is also secreted & this increases urinary excretion by 20% , in the numerator of the clearance formula
- The concentration method measures other substances such as glucose leading to increase 20% in the denominator in the clearance formula
Both the numerator & the denominator cancel each other so creatinine clearance is a fairly good approximation 9f GFR
What is the formula for creatinine clearance?
Ccr= Ucr x V/(PCR)
BUN normally between 10-20mg/dL
low GFR means renal insufficiency
What is renal blood flow?
The volume of blood delivered to the kidney per unit of time
Renal blood flow about 1200 ml/min
What is renal plasma flow?
This is a measure of plasma that flows through the kidney per unit of time
What is effective renal plasma flow?
About 650 ml/min
What is filtration fraction?
Is the fraction filtered & has range of 15%-25%
GFR/RPF= filtration fraction
How is RBF & RPF measured?
Clearance of para-amino hippurate= renal plasma flow (RPF)
Para-amino hippurate(PAH)
-organic anion
- freely filtered
- avidly secreted by peritubular capillaries into proximal tubule
- At low plasma concentrations,the combination of filtration and secretion means that all PAH arriving in the renal plasma is excreted in the urine
RBF= RPF/1-hematocrit
What substances have important clearance?
Clearance of Inulin=GFR
Clearance of creatinine= about GFR
Clearance of PAH= RPF
How are PAH. handling by the kidney?
At low plasma concentrations
Volume of plasma cleared of PAH (CPAH)
= volume of plasma entering kidneys
=renal plasma flow
What is the mass balance equation?
Urine flow rate= 1 ml/min
UPAH= 65 mg/ml
PPAH= 0.1 mg/ml
RPF= CPAH= 65mg/ml/0.1mg/ ml x1 ml/min= 650 ml/min
How can we quantify tubular function?
Filtered load= GFR x Pz
Reabsorp^n= (GFR x Pz)-(Uz x V)
Secret^n= (Uz x V)-(GFR x Pz)
Excretion = Uz x V
Excretion= filtration- Reabsorption+ secretion
What is fractional excretion?
Proportion of the filtered load was excreted= fractional excretion
Fractional excretion of Z (FEz)= rate of excretion of Z/ rate off filtration of Z
What is the fractional excretion of Na used to indicate?
FeNa below 1% suggests prerenal-cause of acute renal failure
FeNa above 2% acute tubular necrosis
Fractional excretion of Na is used to assess if the acute renal failure is due to pre-renal or renal. Causes
What formulas you need to know?
GFR= Urineinulin x V/Pinulin
RPF= UPAH x V/ PPAH
Filtration fraction= GFR/RPF
RBF= RPF/1-hematocrit
