Measuring Fluid Compartments
- Dilution measures a fluid compartment by dividing the amount of a marker remaining in the body by its equilibrated concentration.
- A valid marker must distribute throughout the compartment being measured without leaving that compartment too rapidly.
- Any marker excreted before measurement must be subtracted from the administered amount.
- Tritiated water, D2O, and antipyrine distribute through total body water and estimate its volume.
- Sulfate, inulin, and mannitol remain mainly in extracellular fluid and estimate extracellular-fluid volume.
- Radioiodinated serum albumin and Evans blue bind plasma proteins and therefore estimate plasma volume.
- A compartment marker is useful only when its distribution matches the fluid space being measured, such as total body water, extracellular fluid, or plasma.
- Interstitial-fluid volume is calculated as extracellular-fluid volume minus plasma volume.
- Intracellular-fluid volume is calculated as total body-water volume minus extracellular-fluid volume.
- If 500 mg of mannitol is injected and 10% is excreted, the amount available for dilution is 450 mg.
- Dividing 450 mg by an equilibrated concentration of 3.2 mg per 100 mL gives an extracellular-fluid volume of approximately 14.1 L.
- Because the concentration is given per 100 mL, the calculated volume must be converted from 100-mL units into liters.
- Dilution calculations require matching mass and volume units before division.
Use Hide Bold Terms to cover authored terms, or Hide Every Word to cover every word in the block. Show One reveals the next hidden term or word in reading order without expanding the whole block. In Hide Main, hover a visible term to preview the full bullet.