Question

An individual was injected with 80 mg of inulin and 960,000 counts per min (cpm) of tritium-labeled water (3H20) to determine the volume of various body fluid compartments. After equilibration a blood sample was obtained and the plasma inulin concentration was 0.5 mg% and the plasma activity (concentration) of tritium was 20 cpm/ml. The volumes of which body compartments can be determined?

Answer 1

The measurement of body fluid compartments can be achieved by the dilution of chemical compounds that only circulate and disperse in the region of selected areas in the body. The dilution process is dependent on how the concentration is defined.

Given that:

• the concentration of plasma insulin after equilibrium = 0.5 mg %

∴

Concentration C  = 0.5 mg/100

Concentration C = 0.005 mg/ml

• The mass of insulin = 80 mg

Since the mass amount of the chemical compound(i.e. insulin) and the concentration is known.

The volume of the body fluid compartment can be calculated as:

$\mathbf{volume = \dfrac{\text{mass of the marker }}{concentration }}$

$Volume = \dfrac{80 \ mg}{0.005 \ mg/ml}$

Volume = 16000 ml

Thus, it is known that insulin is generally utilized for the measurement of the extracellular fluid volume and serves as a cell impermeant marker.

As a result;

The volume of the extracellular fluid compartment is 16000 ml.

However, the tritium-labeled water is a good marker for the entire body fluid compartment due to the fact that:

• its diffusion occurs throughout the entire body,
• it is identical to water and;
• the equilibrium concentration is typically easy to measure due to the radioactive characteristics of tritium.

Given that:

plasma activity of tritium = 20 cpm/ml

i.e.

In 1 ml of plasma, 20 cpm of tritium is present.

As such, in 960,000 counts per min (cpm) of tritium-labeled water, the volume of the whole body compartment is:

$\mathbf{= \dfrac{960000}{20} ml \plasma}$

= 48000 ml of plasma

Therefore, we can conclude that the volumes of the body compartment that can be determined are:

• The volume of the extracellular fluid compartment, which is 16000 ml.
• The volume of the whole body compartment, which is 48000 ml

Learn more about body fluid compartments here:

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