Updated: 7/23/2017

Body Fluid Compartments

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  • Body fluid compartments
    • total body water (TBW) is composed of
      • intracellular fluid (ICF)
        • ~2/3 of TBW
      • extracellular fluid (ECF) (~1/3 of TBW) which is composed of
        • interstitial fluid
          • ~ 3/4 of ECF
          • interstitial fluid is fluid in which cells are bathed
        • plasma
          • ~1/4 of ECF
          • plasma volume is measured by radiolabeled albumin
    • solute composition of fluid compartments
      • intracellular fluid
        • potassium is abundant in the ICF
          • mnemonic: HIKIN = HIgh K+ INtracellularly
      • extracellular fluid
        • sodium and chloride is abundant in the ECF
        • ECF is measured by inulin
    • "60-40-20 Rule"
      • 60% of body weight is total body water
      • 40% of body weight, or 2/3 of total body water, is intracellular fluid (ICF)
      • 20% of body weight, or 1/3 of total body water, is extracellular fluid (ECF)
  • Osmosis
    • describes the movement of water through a semipermeable membrane
      • water moves from an area of high water concentration (or low solute concentration) to an area of low water concentration (or high solue concentration)
    • units
      • osmolarity (mOsm/L)
        • solute concentration per liter of solvent (e.g., water)
      • osmolality (mOsm/kg)
        • solute concentration per kilogram of solvent
    • effective osmole
      • a solute that does not cross the cell membrane (e.g., protein)
        • this creates an osmotic force that "pulls" water into its compartment
Osmolarity and Volume Changes Under Certain Conditions
Extracellular Fluid
Intracellular Fluid
Blood Osmolarity
Isotonic fluid loss
  • No change
  • No change
Hypotonic fluid loss
Isotonic fluid gain
  • No change
  • No change
Hypotonic fluid gain
Hypertonic fluid gain

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