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Flashcards in Physiology Equations/Definitions Deck (23):
1

Normal osmolarity

290 mOsm/L

2

Equation for plasma osmolarity

2 x Plasma Na+ + Glucose/18 + BUN/2.8 Na in mEq/L, glucose and BUN in mg/dL

3

Clearance

Volume of plasma completely cleared of a substance per unit time; mL/min

Cx = (UF x [U]x)/[P]x

4

Clearance ratio

Clearance ratio = Cx/Cinulin

Compares clearance of a substance to clearance of inulin.

< 1.0 = not completely filtered, or reabsorbed

> 1.0 = actively secreted

5

Molecule that measures GFR (glomerular marker)

inulin - not secreted or reabsorbed

6

Molecule with the highest renal clearance

para-aminohippuric acid ( PAH) - actively secreted and not reabsorbed. Extraction ratio ~ 90-95%.

7

Renal plasma flow

RPF = ([U]PAH x UF) / ([RA]PAH - [RV]PAH)

in mL/min

8

Effective RPF

ERPF = ([U]PAH x UF) / [P]PAH

in mL/min

Clearance of PAH. Underestimates by 10%. Can be divided by extraction ratio of PAH to get RPF.

9

RBF

RBF = RPF/(1-Hct)

in mL/min

Normal: 1.25 L/min or 1800 L/day 

10

Filtered load

Filtered load = GFR x [P]x

in mg/min

Amount of a substance filtered into Bowman's space per unit time

11

GFR

Normal = 180 L/day, 120 mL/min

Same as clearance for inulin. Estimated with creatinine clearance.

12

Filtration fraction

FF = GFR / RPF

13

Measuring volume of body fluid compartments

Dilution principle V = (quantity injected/concentration) - volume of dye injected

14

Blood volume

BV = PV/(1-Hct)

15

Major ions in plasma, ISF, and ICF

Plasma = Na+, Cl-, HCO3-, Pr- ISF = Na+, Cl-, HCO3- ICF = K+, Phosphates-, Pr-, HCO3-

16

Mass flow

Expression of solute movements as a function of time. Usually mg/min. Mass flow = concentration x volume flow

17

Mass balance in a nephron

filtration + secretion = excretion + reabsorption

18

Excretion rate

Excretion rate = UF x [U]x

in mg/min

19

Extraction ratio

Percentage of a substance removed from the plasma. High with secretion, low with reabsorption. Extraction ratio of x = (Ax-Vx)/Ax

20

Reabsorption or secretion rate

Reabsorption/Secretion = filtered load - excretion

in mg/min

If positive, net reabsorption

If negative, net secretion

21

Free water clearance

Free water clearance is extra water excreted to make urine hypotonic

CH2O = UF - Cosm

CH2O = UF - (1 - Uosm/Posm)

in mL/min

If positive, free water is excreted

If negative, free water is reabsorbed

22

Cosm and Cwater

Osmolar clearance: water needed to excrete solutes at the concentration of solutes in plasma, obligatory water excretion

Urine vol = Cosm + Cwater

Cosm = Uosm/Posm x V
Normal Cosm = 2 mL/min

23

Fractional Excretion of Na

Fraction of filtered load that is excreted into the urine

FENa = ( Pcr x Una )/( PNa x UCr