Renal Clearance - Lecture 2 Flashcards
What are the 3 general processes of Renal Function?
- Glomerular Filtration
- Reabsorption of the substance from tubular fluid back into the blood
- Secretion of the substance from blood into tubule fluid
What is the equation for load?
- Kidney input Load
- Kidney Output load
Load (mg/min) = Concentration (mg/mL) * Flow (mL/min)
Kidney input = Pax * RPFa (arterial plasma concentration X Renal plasma flow)
Kidney Output = (Pvx * RPF v) + (Ux * V)
V= urine flow rate Ux = urine concentration of x
What is the principle of the conservation of mass?
Kidney inputs must equal kidney outputs
Pax * RPF a = (Pvx * RPF v) + (Ux * V)
When is RPFa > RPFv?
If V (urine flow rate) is greater than ZERO
What is true for any substance that is neither synthesized nor metabolized?
The amount that enters the kidneys is equal to the amount that leaves the kidneys in urine PLUS the amount that leaves the kidneys in renal VENOUS blood.
What do the following mean:
- Filtered Load
- Secreted Load
- Reabsorbed Load
- Excreted Load
What does E equal??
F+ S = R + E
- Filtered load is tubular INPUT
F = Pax * GFR - Secreted load is tubular input
- Reabsorbed load is tubular output
- Excreted load is tubular output
E = Ux * V
E= F+ S - R
What is the concept of clearance?
Renal Clearance emphasizes the excretory function of the kidneys
- ONLY THE RATE AT WHICH A SUBSTANCE IS EXCRETED
- not the rate of return to the circulation in the renal vein
What is Pax proportional to?
Pax is proportional to Ux * V
What is clearance defined as?
The volume of plasma completely cleared of any substance in one minute
How much does the kidney receive of the Cardiac Output?
How much of the Renal Plasma flow is filtered?
25%
20%
What is the general formula for clearance?
Cx = (Ux * V) / Px
What do the following mean
if Cx < GFR
Cy = GFR
Cz > GFR
- Substance is filtered and reabsorbed
- Substance is only filtered
- Substance is filtered and secreted
What are the Cx Cy or Cz for
- Glucose
- Inulin
- Creatanine
- PAH & Organic Dyes
- minimum Cx for glucose & Proteins - substance is filtered & reabsorbed (not usually excreted)
(Fx > 0 & R >0) for proteins Fx = 0
- mid-range Cy=GFR for Inulin & Creatanine
- substance is ONLY filtered
(Fy >0 Sy = Ry = 0)
- Maximum Cz = RPF for PAH and organic dyes
- substance is filtered and secreted
(Fz > 0 & Sz>0)
What does the following mean:
F= E
If any substance in the plasma is freely filtered by glomerulus, but is not secreted nor reabsorbed, the clearance would be an accurate measure of GFR
Filtration thus is the same as Excretion
What are the advantages and disadvantages of using exogenous Inulin to determine GFR?
- Freely filtered fructose polymer
- Neither reabsorbed nor secreted
- Not bound to plasma proteins
- Does not alter renal functions
- Easily & Accurately quantified in plasma & urine
- Inulin must be infused to obtain constant blood titers (disadvantage)
- Inulin is expensive (disadvantage)
Cinulin = GFR of inulin
True or False: All the Creatinine at the glomerulus is filtered.
FALSE!
Not all the creatinine is filtered at the glomerulus. Only 15-20% is filtered.
What are the advantages/Disadvantages for using Creatanine for GFR?
- End product of skeletal muscle metabolism
- Freely filtered
- Not reabsorbed
- Not bound to plasma proteins (no hidden reserve)
- Neither produced nor consumed by kidneys (inert)
- Does not alter renal functions
- Easily quantified in plasma & urine
- BIG ADVANTAGE - creatinine is constantly being “infused” into the plasma
- FREE! does not need to be purchased
- Plasma concentrations are overestimated by 10%
What are the main advantages of using Creatanine over Inulin to measure GFR?
- BIG ADVANTAGE - creatinine is constantly being “infused” into the plasma
- FREE! does not need to be purchased
- Plasma concentrations are overestimated by 10%
How does the amount of creatine filtered compare to the amount excreted?
Amount of creatine filtered < the amount of creatine excreted (by 10%)
(filtration is about the same as Excretion)
What does it mean if Clearance * Plasma concentration of Creatinine matches the creatine production rate?
- Decreased functional glomeruli
- Decreased GFR
- Decreased Clearance of Creatinin
- Increased plasma concentration of Creatinine
- decreased GFR can be estimated by increased Pcr
What is the relationship of Pcr and BUN?
Pcr is more stable than BUN
PCr is a more accurate estimator of GFR than BUN
How does GFR* Pcr for a lean person compare for a more muscular person?
it is LESS for a lean than for a muscular person
If the GFR falls from 120 to 60, how does the Pcr change?
Increases from 1 to 2 mg/dL
Describe the important aspects of PAH
- Small molecular weight
- Freely filtered and secreted by the PROXIMAL TUBE
- Much more is removed than what is filtered
- at low plasma concentrations, almost all PAH is removed from the plasma & excreted into the urine
- Its clearance is nearly as great as the renal plasma flow
What is the equation for the amount of PAH excreted?
PAH filtered + PAH secreted = PAH excreted
What are the advantages/disadvantages of PAH for determining RPF?
- PAH is freely filtered
- Not reabsorbed (R=0)
- Actively and efficiently secreted (S>0) by 90 %
Why is PAH called the approximate or EFFECTIVE Renal Plasma Flow? (RPF)
Because it UNDERESTIMATES true RPF by about 10 %
- when calculating RPF with PAH, must correct for this 10% (add = 110% or 1.1 multiplied by the RPF achieved from PAH)
RPF = 1.1 * ERPF (mL/min)
How is RPF converted to Renal BLOOD flow? (RBF)
RBF = RPF/ (1-hematocrit)
hematocrit = 45% (.45)
How does one get stroke volume from RBF?
RBF/CO = Stroke volume (25%)
What is the RPF?
The fraction of RPF filtered across the glomerular capillaries
What is a normal Filtration Fraction? How does one get this value?
What happens to the reabsorption in the proximal tubule if filtration is increased? Decreased?
20%
FF = GFR/ PRF
or Cin/Cpah
If Filtration Fraction is increased: reabsorption at the proximal tubule increases due to increase protein concentration in the peritubular capillaries
If Filtration Fraction is DECREASED: reabsorption at the proximal tubule DEcreases due to decreased protein concentration in the peritubular capillaries
What happens to BUN and Plasma Creatine when GFR increases?
they DECREASE
What is the relationship of Plasma Creatine and GFR? Why is this called a “steady state” relationship?
INVERSE relationship!
As GFR increases, plasma concentration decreases
- steady state because once the creatine matches the change in GFR, the values stay constant
EX: if GFR is decreased by 50%, and the original Plasma Creatine is 1 mg/dl - the Plasma creatine will increase to 2 mg/dL to compensate for the decrease in GFR NO MORE CHANGES once this occurs
What is the max value for Clearance?
RPF!
