Question

Using these data-Calculate Extracellular Volume (ECV) in liters :

Amount ofD20 injected, corrected for loss = 40 mg

Plasma D20 concentration = 1 mg / L

Amount of inulin injected, corrected for loss = 20 mg

Plasma inulin concentration = 2 mg/L

Amount of Evan's Blue injected, corrected for loss = 2.5 mg

Plasma Evan's Blue concentration = 1 mg/ L

Hematocrit (Hct) = 40%\

(please show your work)

Answer 1

This is the dye dilution technique which depends upon the principle of Volume of Distribution. In this a known quantity of suitable dye / indicator is injected to a unkown volume. After equilibrium a sample is withdrawan and concentration of the indicator / dye is measured in the sample. The volume of the fluid compartment is measured by using the following -

V = Q-e / c

Where

V = Volume of the compartment.

Q = Quanitiy of indicator ,

C = concentration of the indicator,

e = amount of indicator which is either lost or metabolised.

NOW, In this question we have been asked to calcluate the volume of ECF. Hence, an indicator which will distribute only in the extracellular fluid will be the best indicator for calculation of ECF volume.

Inuin is Distributed only in the Extracellular fluid compartment and hence the BEST INDICATOR FOR DETERMING ECF VOLUME:

Amount of inulin injected, corrected for loss = 20 mg

Plasma inulin concentration = 2 mg/L

ECF volume(V) = Q-e / C

here Q = 20 mg, e = 0 (zero - as insulin is not being excreted or metabolised), C = 2 mg/L

Hence t

V = 20/2 = 10 Liter

Hence the extracellular fluid volume in this case is 10 Liter.

COMPARTMENT	INDICATOR USED
Total body water	D2O is mostl frequently used other - Tritium oxide, antipyrine
ECF volume	Insulin is best others - sucrose, mannitol, raffinose, radioisotope like Na+, Cl- ect