Renal Equations

Question 1

equation for GFR

Answer 1

GFR = K_f x ΔP

• Kf = filterability coefficient
  
  • < 15 A = freely filterable
  • 15-35 A = inverse relationship w/ size and filterability
  • > 35 A = no filterability

Question 2

equation for filtered load/rate of filtartion (FL)

Answer 2

rate at which solute is filtered per unit time (mEq/min)

FL = GFR x P_x (if < 15 A)

FL = GFR x P_x x filterability quotient (if 15 - 35 A)

Question 3

equation for filtration fraction (FF)

Answer 3

% of plasma being filtered

FF = GFR/RPF

Question 4

equation for renal blood flow (RBF)

Answer 4

related to RPF through hematocrit

RBF = RPF/(1-hct)

~45% in males

~42% in females

Question 5

equation for net transport of solute (TR)

Answer 5

TR = FL (rate of filtration) - ER (rate of excretion)

= (GFR x P_x) - (U_x x V)

Question 6

equation for excretion rate

Answer 6

ER = U_x x V

Ux = urine concentration of solute

V = rate of urine flow

Question 7

equation for renal clearance

Answer 7

volume of plasma cleared of a particular solute by the kidneys per unit time

C = (U_x x V) / [P]_x

C = clearance (ml/min)

[U] = urine concentration (mg/ml)

V = urine flow rate/min (ml/min)

[P] = plasma concentration (mg/ml)

Question 8

distribution of body water

Answer 8

• males: TBW = 60% weight in kg
• females: TBW = 50% weight in kg
• ICW = 2/3 TBW
• ECW = 1/3 TBW
  
  • ECW = ISF + PV
    
    • ISF = 3/4 ECF
    • PV = 1/4 ECF
      
      • Blood volume (BV) = PV / (1-hct)

Question 9

dye dilution technique

Answer 9

compartment volume = (amount injected - amount excreted) / ([substance] after equilibrium (mg/L))

• measures TBW, ECW, PV directly