•
© 2010 Mark Tutt le
Renal Physiology Equations
Cx =
(U Jx ·V
Cx
X i sany substance not m etabo lized by the
kidney
serum concentration
C1n-ulln = Cereattntne= GFR
-
(PJx
Clearance
excr etio n rate
x is any substance notm etabo lized by the kid ney
ml/ min
C.= Clearanceof substance x
Flx
Amount
excreted
or secreted .
c:, Act uall y is secretedin small amounts by
[Ulx = Urine concen tra ti on of x
V = Urine flow
(P}x = Plasmaconcentrato
i n of x
Filtered Load
Creatinine is filtered (20%}, but not reabsor bed
PT
overest imates GFR by10-20%
lnulin is filtered, but not reabsorbed/secreted.
Flx = GFR · (XJser«1n
mg
Ex= amt. filte red + amt. secreted - amt. reabsorbed
Amount of substance X t ha t is filte re d
th rough the glo merulus
Amount of substance X t h at is found
mg
in the urine
Ex
Of the filtered amount of, X what ratio
is found in the urine.
Fractional
Excretion
Ex amount excreted
FEx= Flx = amount filtered
FEx
Cx
- C 1null n
%
Determ ines net reabsorb ti on/ secr eti on:
Fltx < 100% net reabsorption ofX
FEx < 100% net secretion of X
.
.
In r enal fail ure (olig uria):
.
.
Net
rea bso rpt ion
or secretion
Excretion
Rate
Fl x - Ex
mg
ERx = (U]x · V
mg/min
GFR
FF = RPF
%
ERx
Filtration
Fraction
FF
Renal Plasma
Flow
RPF "' CPAH =
RPF
Renal Blood
Flow
RBF =
RBF
Free-Water
Clearance
CH20
Cockroft-
RPF
1 - Hematocrit
Uosm
cH, o = V. Cosm = V - V
p
osm
Cer =
72 · [PJereatl11l11e
A=l for males, 0.85 for fe male s
Hende rson-
(HC0 3 J
pH =p k + log [H AJ = p k + log 0.03p. C0 2
H asselbal
ch
ml/ min
ml/ min
ml/ min
(A
l
Rate at which X accumulates in the
urine
No rmal is 0.2
• u n d eres t i m a t e sRPF by 10%
(because i t' s reall y only 90% excreted)
1 -Hct is fra ction o f blood volume that
is plasma
If positive, free water is excreted
If negative, free water is reabsorbed
De term ines if the k i d n ey is co n centra ti ng o r
d il ut ing u r i ne
A · (140 - age)(kg)
Gault
If positive, there is net reabsorption of X.
If negative, there s net secret ion of X.
PAH is 100%* excreted
[U JP AH · V
[PJPAH
FENo• < 1% Prerenaldisease
FENo• > 2% Ac ut e tub ular n ec ro si s
ml/ min
.
© 20 10 M ark Tutt le
•
Fluid Compartment Markers
.
D,O
Titrat ed H20
TBW:
Total Body Water
.
ICF:
Int racellular Fluid
TBW - ECF
40%
Extracellular Fluid
.
.
.
20%
ECF:
60%
Sulfate
lnulin
Mannitol
ISF: In terstit ial Fluid
ECF- Plasma
.
Plasma
.
.
.
.
Risa
Evans blue
Radioactive albumin
•
Normal Serum Levels
[BUN)
4-8 mmol/ l
BUN:Creat.inine
10
Blood pH
7.38 - 7.42
pCO,
40mmHg
[HCO,·)
24 mEq/ l
1'
indicates azotemia
> 20:1 Prerenal azotemia
1:1
Indicates acidosis/alkalosis
Serum Anion Gap
INa+]-([HCO,]·+[Cl·l)
Unmeasured ionsincludephosphate,
citrate, sulfat e, a nd prot ein.
1'
12 mEq/L
(8-16 mEq/ L)
to replace [HCO,·] ,!, in metabolic acidosis
If anion gapis increased, there is anincreasein an unmeasured
ion, u sual ty(phosphate, lact ate,-13 hy d ro xybutyrate)
If anion gapis normal in metabolic acidosis, Cl • haslikely taken
the place of HC03·, called hyperchloremic metabolic acidosis.
Urinary Anion Gap
[Na+]+[K•]- [CI·]
Unmeasured ions include ammonium.
Acute renal failure
Near zero or pos it ive
In metabolic acidosis, th e excretion of theNH4• (whichis
excreted with c1· } should increasemarkedly if renal acidification
is intact. Be<:ause of th erisein urinaryc1· , the urine anion gap
which is also called the urinary net charge,becomesnegative,
ranging from •20 to more than•SOm eq/L The negativevalue
occurs because the Cl· concentration now exceeds the sumtotal
of Nat and K .
In contrast i f there is animpairment in kidneyfunction resulti ng
in an inability to increaseammonium excretio n (i.e. Renal
Tubular Acidosis}, thena·ions willnot beincreased in the urine
and theurine anion gapwill not beaffectedand willbepositive
or zero.