As always, be sure to show your calculations. First, let’s consider total body w

Question

As always, be sure to show your calculations.

First, let’s consider total body water. The best way to estimate this is to use D2O or 3H2O. Your patient is a 45 year-old 62 kg female; you have injected 3.1 mg of D2O, and waited a brief interval for full distribution. Assume that the quantity of heavy water in the urine is similar to all other fluid compartments. You measure a D2O concentration of 10-4 mgmL-1 in her urine. What is her total body water?

Second, let us measure plasma volume and blood volume. For plasma volume, use an azo dye like Evan’s blue that avidly binds to albumin. Same patient, some new data: her hematocrit is 42. Ignoring all the pitfalls of this method, such as inaccuracy of hematocrits and continuous disappearance of albumin from the vascular space, you determine that the 10 mg of Evan’s blue administered to the blood stream results in a concentration 3.8310-3 mgmL-1. What is her plasma volume?
And what is her blood volume?

Finally, there really is no way to directly measure interstitial fluid volume, so we shall approach this slightly differently. Since we know plasma volume and total body fluid volume, if we can measure the extracellular fluid volume (ISFV + PV], we can determine the ISFV. We inject 10 mg of a crystalloid tracer (inulin) [not perfect because, due to limitation in its movement, it underestimates the ECFV], and, after about 24 hours, measure a concentration in her plasma of 0.794 mgmL-1. What is her interstital fluid volume?
And her intercellular fluid volume?

Explanation / Answer

Hi,

1. The question here is an example of indicator dilution method to determine the size of the body fluid. The simple formula to determine the total fluid is, Volume = Amount injested / Concentation of the indicator.

In this case, the volume = 3.1 mg / 10-4 mg/ml

Total body water(TBW) Volume = 3.1 X 10-4 = 31000ml = 31 L.

2. Blood plasma volume can be found out by similar approach.

Blood palsma volume = 10 mg / 3.83 x 10-3 = 2.6 L.

Blood volume = Plasma volume / (1 - Hematocrit) : Hematocrit = 42% = 0.42

= 2.6 / (1-0.42)

= 4.4 L.

3. ECF = 25% of plasma.

Therefore, ECF = 2.6 /4 = 0.65L

The intracellularl fluid volume = Total body water - ECF

= 31 - 0.65

ICF = 30.35 L

Interstitial fluid = ECF - plasma

2.6 - 0.65 = 1.95

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