Physiology equations

Question 1

Clearance

Answer 1

C_X=[U]x∙V)/[P]x= (excretion rate)/(serum concentration)

units=ml/min

x is any substance not metabolized by the kidney

Cx = Clearance of substance x (ml/min)
[U]X = Urine concentration of x (mmol/L)
V = Urine flow rate (ml/min)
[P]X = Plasma concentration of x (mmol/L)

Question 2

GFR

Answer 2

CIn=([U]In∙V)/[P]in =(excretion rate)/(serum concentration)

units ml/mins

Question 3

GF

Answer 3

GFR = k[(Pgl + pieBC) – (PBC + piegl)]

GFR = k[NFP]

K = filtration coefficient
P = Hydrostatic pressure
pie = oncotic (colloid osmotic)
pressure Within the glomerular capillaries (gl) or Bowman’s capusule (BC)

Question 4

Filtered load FLx

Answer 4

FLx=GFR⋅[X]serum

units mg/min

Quantity of substance X that is filtered through the glomerulus over some period of time

Question 5

Excretion Rate ERx

Answer 5

ERx=[U]x∙V

units mg/min

Question 6

Amount excreted

Ex

Answer 6

Ex= amt. filtered – amt. reabsorbed + amt. secreted

units mg/min

Question 7

Fractional Excretion
FEx

What does this determine?

What does it mean in renal failure?

Answer 7

FEx=Ex/FLx =(amount excreted)/(amount filtered)=Cx/Cinulin

Determines net reabsorption/secretion:

• 𝐹𝐸” < 100% → net reabsorption of X
• 𝐹𝐸” < 100% → net secretion of X

In renal failure (oliguria):

• 𝐹𝐸Mna <1%→Prerenal disease
• 𝐹𝐸MNa >2%→Acutetubularnecrosis

Question 8

Net reabsorption or secretion

What is positive and negative?

Answer 8

FLx-Ex

nits mg/min

If positive, there is net reabsorption of X.
If negative, there is net secretion of X.

Question 9

Filtration fraction normal FF

What is normal filtred Fraction?

Answer 9

FF=GFR/RPF

Normal is 0.2 (or 20%)

Question 10

Renal plasma flow RPF

Answer 10

RPF≈C_PAH=([U]PAH∙V)/[P]PAH
nits ml/min

PAH is 100%* excreted
*Underestimates RPF by 10% (because it’s really only 90% excreted)

Question 11

Renal Blood Flow

RBF

Answer 11

RBF=RPF/(1-Hematocrit)

units=ml/min

Question 12

Free-Water Clearance
CH2O?

What does this determine?
What is positive and negative?

Answer 12

C(H2O)=V∙Cosm=
V-V(Uosm/Posm)

ml/min

If positive, free water is excreted If negative, free water is reabsorbed
Determines if the kidney is concentrating or diluting urine

Question 13

Cockroft-Gault?

What is serum creatinine proportional too?

Answer 13

C_Cr=(A⋅(140-age)(kg))/72⋅[P]Creatinine

A=1 for males, 0.85 for females

Serum creatinine is inversely proportional to GFR and can be used to determine an eGFR without having to do a full clearance study.
Age in years; [P]creatinine in mg/dL

Question 14

What is henderson hasselbach?

Answer 14

𝑝𝐻=𝑝𝑘+𝑙𝑜𝑔[𝐴-]/[HA]=𝑝𝑘+𝑙𝑜𝑔 ([𝐻𝐶𝑂-3]/ 0.03 *𝑝𝐶𝑂2)