Renal Equations

Question 1

Renal Clearance

Answer 1

Cx = V (Ux) / Px

V - urine flow rate
Ux - conc of x in urine
V (Ux) together - urine excretion rate

Question 2

GFR

Answer 2

**clearance of creatinine used to estimate GFR (slight overestimate)

**clearance of inulin used to calculate GFR (research)

GFR = Kf (Pgc - Pbc) - (PIgc - PIbc)

Question 3

What is the relationship between creatinine in plasma and GFR

Answer 3

INVERSE (less plasma creatinine means more GFR)

But not linear (large changes in GFR produce only small changes in creatinine)

Question 4

eRPF

Answer 4

“Effective renal plasma flow”

Estimated by PAH clearance b/c PAH is totally excrete between filtration and secretion

Slight underestimate of RPF

Question 5

Renal Blood Flow

Answer 5

RBF = RPF / (1 - Hct)

Question 6

Filtration Fraction

Answer 6

GFR / RPF

Question 7

What increases FF?

Answer 7

• efferent arteriole constriction (angio II)
• lower oncotic pressure in plasma
• dehydration (because RPF decreases way more than GFR decreases)

Question 8

What decreases FF?

Answer 8

• higher oncotic pressure in plasma

- ureter constriction (backup into tubules so dec GFR from inc hydrostatic pressure of Bowman’s capsule)

Question 9

Filtered Load

Answer 9

GFR x Px

Question 10

Fractional Excretion of Na

Answer 10

(Pcr X Una) / (Ucr X Pna)

Question 11

Henderson-Hasselbach Equation

Answer 11

pH = 6.1 + log ([HCO3-]/.03 Pco2)

Question 12

Winter’s Formula

Answer 12

PCO2 = 1.5 [HCO3-] + 8 +/-2

Represents range of what the PCO2 should be to compensate for metabolic acidosis

If actual PCO2 is lower than not enough comp … additional resp alkalosis

If actual PCO2 is higher than too much comp … additional respiratory acidosis