Renal 2; Clearance Flashcards
What are the 3 basic renal units ?
Concentration (mg/ml)
Flow(ml/min
Load(mg/min): concentration x flow
What 3 key factors contribute to renal concentration?
Pax = arterial plasma concentration of x
Pvs =venous plasma concentration of x
Ux= urine concentration of x
What 4 key factors contribute to renal flow?
RBF= renal blood flow
RPF=renal plasma flow
GFR =glomerular filtration rate
V*= urine flow rate
How do we calculate kidney input load from arterial plasma concentration and renal plasma flow?
Kidney Input Load = Pax * RPFa
How do we calculate kidney output load from urine concentration, urine flow rate ,renal plasma flow and venous plasma concentration?
Kidney Output load = (Pvx * RPFv) + (Ux * V*)
Principle of Conservation of mass
For balance, Kidney input = kidney output
Pax * RPFa = (Pvx * RPFv) + (Ux * V*)
RPFa is greater/less than RPFv if V* is greater than 0?
Greater
What are the 4 basic renal loads and do they each represent input or output?
(F) filtered load - tubular input (S) secreted load - tubular input (R) reabsorbed load - tubular output (E)excreted load - tubular output E = F +S-R
How does filtered load F relate to Pax and GFR?
F= Pax * GFR
How does excreted load relate to urine concentration Ux and urine flow rate V*?
E= UxV
What is clearance?
The volume of plasma completely cleared of any substance in one minute. a flow, not a load or concentration.
What are the minimum and maximum values of clearance?
min is 0. max is RPF.
How is clearance a “virtual flow”?
Few substances are completely cleared from renal blood in one pass because the kidney receives about 25% of CO, and filters only 20%
What is the general formula for clearance?
Load delivered to kidney = load removed from kidney in urine (But ont venous plasma; ignore because venous blood returned to body)
Cx = Ux x V*/Px
What happens to a substance if
Cx <GFR?
x is filtered and reabsorbed
What happens to a substance if
Cx = GFR?
x is only filtered
What happens to a substance if
Cx > GFR?
x is filtered and secreted.
What is the range of clearance values and where would the following compounds be found along that range? glucose proteins inulin creatinine PAH organic dyes
minimum Cx = 0 for glucose (Fx > 0 & Rx > 0) and proteins (Fx = 0)
mid-range Cy = GFR for inulin and creatinine (Fy > 0 & Sy = Ry = 0)
maximum Cz = RPF for PAH and organic dyes (Fz > 0 & Sz > 0)
What kind of substance would have a clearance that would accurately measure GFR?
A substance that was freely filtered by the glomerulus but was neither secreted nor reabsorbed.
F = E (S=0 R =0)
What is the clearance equal to for a substance that is filtered through the glomerulus but neither reabsorbed or secreted?
Cx = GFR
What are the advantages for using inulin to determine GFR?
Freely filtered neither reabsorbed or secreted not bound to plasma proteins not produced/consumed by kidneys nontoxic quantifiable
What are the disadvantages of using inulin to measure GFR?
Must be infused to obtains constant measurements
Expensive
What advantages does creatinine have over inulin?
constantly being infused into plasma without assistance
“Free” in that the body naturally produces it and doesn’t need be purchased
What is the key disadvantage of creatinine?
Plasma concentrations of creatinine are overestimated by about 10%
Clearance of Inulin
C (inulin) = U(inulin)V/P(inulin)
C(inulin) = GFR
How is the filtration and excretion of creatinine related?
amount excreted is 10% less than amount filtered
Fcr ~ Ecr
Clearance of creatinine
Ccr = Ucr * V*/Pcr
Ccr ~ GFR
How is Blood-Urea-Nitrogen (BUN) related to Pcr?
BUN is derived from catabolism of nitrogen-containing proteins. It is roughly Pcr x 10, and is not as accurate an estimator of GFR because just eating a meal can raise BUN
What happens when we get a decrease in functional glomeruli (renal failure)
Decreased GFR –> decreased Ccr –> increased Pcr
GFR * Pcr matches constant creatinine production rate
How does clearance of para-aminohippurate equal effective renal plasma flow? What is the equation for PAH clearance?
PAH is filtered and secreted but not reabsorbed.
So F + S = E and R = 0.
RPF = Ux * V* / (Pax - Pvx) (EXACT)
ERPF = Ux x V*/(Pax) (APPROXIMATE)
Cpah = ERPF = Upah * V*/Papah
What is ERPF and its relation to RPF
Effective Renal Plasma Flow
RPF = 1.1 ERPF (corrects for 10% of PAH not secreted in vasa recta capillaries)
How do we convert RPF to RBF?
RBF = RPF / (1-Hct) where Hct is hematocrit which is typically 45%
How do you find Filtration Fraction (FF)
FF = GFR/RPF = Cinulin/Cpah
Normaly FF = 20%
Rank the following from greatest to least: (based on typical values) GFR RBF RPF V*
RBF»_space; RPF»_space; GFR»_space; V*
