Renal Clearance Flashcards
What are the units of clearance?
Volume per unit time
What are the 3 processes in the nephron?
Glomerular Filtration
Resorption of the Substance into Blood
Secretion of the Substance into Tubule
What are the renal units for concentration?
mg/mL
What are the renal units for flow?
mL/min
What are the renal units for load?
mg/min
Pax
Concentration x in Arterial Plasma
Pvx
Concentration x in Venous Plasma
Ux
Concentration x in Urine
RBF
Renal Blood Flow
RPF
Renal Plasma Flow
GFR
Glomerular Filtration Rate
V
Urine Flow Rate
What is the kidney input load equal to?
Pax + RPFa
What is the kidney output load equal to?
(Pvx * RPFv) + (Ux * V)
If urine flow rate V>0mL/min then how are the values of the RFPa and RFPv affected?
RPFa > RPFv
What is the equation for conservation of mass in regards to renal function?
Pax + RPFa = (Pvx * RPFv) + (Ux * V)
What are the loads that enter the tubule?
F - filtered load
S - secreted load
What are the loads that leave the tubule?
R - reabsorbed load
E - excreted load
What is the filtered load equal to?
Pax * GFR
What is the excreted load equal to?
Ux * V
What is clearance?
It is defined as the volume of plasma completely cleared of any substance in one minute
What is the range of clearance?
0 to RPF (maximum)
What is the RPF in terms of the cardiac output?
RPF is around 25% of the cardiac output
How much of the RPF does the kidney filter?
20% -> this is the GFR
What is the formula for clearance?
Cx = Ux * (V/Px)
Px - plasma concentration of x
Why is the venous return ignored in clearance?
Venous blood is returned to the body
What happens if Cx < GFR?
Substance x is filtered and reabsorbed
What happens if Cx = GFR?
x is only filtered
What happens if Cx > GFR?
x is filtered and secreted
What is the clearance range for glucose and proteins?
Cx = 0 usually
What is the clearance range for inulin and creatinine?
Cx = GFR
What is the clearance range for PAH and organic dyes?
Cx = RPF
What substances can give an accurate measurement of GFR?
Substances that are filtered by the glomerulus but neither secreted nor reabsorbed would be an accurate measurement of the GFR.
F = E in this case as S and R = 0
What are the advantages of using inulin to determine GFR?
- It is neither reabsorbed or secreted so S and R = 0
- Neither produced or consumed by kidneys
- Easily measurable
- Nontoxic
What are the disadvantages to using inulin to determine GFR?
- It must be infused constantly
- It is expensive
What are the advantages to using creatinine to determine GFR?
- It is a natural metabolic product of skeletal muscle metabolism
- Not reabsorbed so R=0
- Neither produced or consumed by kidneys
- Nontoxic
- Easily measured
- Constantly being infused naturally
- “Free” so it is cheap
What are the disadvantages of using creatinine to determine GFR?
-10% is secreted so S>0
Which method of measuring GFR is more accurate?
Inulin
Which method of measuring GFR is more accessible?
Creatinine
How does the value of the GFR measured by the creatinine method differ from the true value?
It is overestimated by around 10%
What is BUN and how is it used?
It is the blood urea nitrogen and it is used to roughly estimate GFR.
What is the relationship between GFR and PCr?
As the GFR decreases, the PCr will increase as the excretion is falling while the production rate remains constant.
Why is PAH a good estimation for RPF?
It is not normally found in the body and it is freely filtered and secreted by the proximal tubule and more is removed than what is filtered.
R=0 as none is reabsorbed.
What is the RPF equal to?
RPF = RBF * (1-Hematocrit)
