USMLE Step 1 Renal Physiology 1 Review 58 10 The Indicator Dilution Principle

USMLE Step 1 Renal Physiology 1 Review 58 10 The Indicator Dilution Principle

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Play USMLE Audio MP3 58 10 The Indicator Dilution Principle Below

Begin 58 10 The Indicator Dilution Principle Transcription

What principle is used to measure the volume of water in the fluid compartments of the body? The indicator dilution principle.

What does the volume of the fluid compartment being measured equal in terms of the amount of the marker substance used for the measurement and the concentration of the substance?

  • The volume of the fluid compartment equals the amount of the marker substance administered minus the amount of marker substance lost from the compartment. This is divided by the concentration of the marker substance in the body fluid.

Alphabetically, list three ways that a marker substance can be lost from or leave the fluid compartment being measured. I’ll cue you with the first letter of each.

First, E.

  • Excretion.

Second, M.

  • Metabolism

and third, V.

  • Vaporization.

What is one substance that can be used as a marker for measuring plasma volume. Its acronym is R-I-S-A, RISA.

  • Radioiodinated human serum albumin or RISA, can be used to measure plasma volume.

What dye can also be used to measure plasma volume?

  • Evans blue dye.

Name two fluid compartment volumes that can’t be measured directly.

  • Interstitial fluid volume and intracellular fluid volume can’t be measured directly.

How is interstitial fluid volume calculated?

  • Interstitial fluid volume is calculated by subtracting plasma volume from extracellular fluid volume.

And how is intracellular fluid volume calculated?

  • Intracellular fluid volume equals total body water minus extracellular fluid volume.

Student doctor, please pause the tape and summarize what we’ve discussed about the indicator dilution principle.

  • The indicator dilution principle is used to measure the volume of water in a fluid compartment. The volume of the fluid compartment equals the amount of the marker substance administered minus the amount of marker substance lost from the compartment. This is divided by the concentration of the marker substance in the body fluid. Three ways that a marker substance can be lost from or leave the fluid compartment being measured are excretion, metabolism, and vaporization. Radioiodinated human serum albumin and Evans blue dye can both be used to measure plasma volume. Neither interstitial fluid volume nor intracellular fluid volume can be measured directly. Interstitial fluid volume is calculated by subtracting plasma volume from extracellular fluid volume. Intracellular fluid volume equals total body water minus extracellular fluid volume.

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