Fluid and Electrolytes

Fluid and Electrolytes

Normal physiology

  • Maintenance of homeostasis
  • Composition of fluids and electrolytes kept within narrow limits
  • Water content varies with age, gender, and fat content

Fluid Compartments

  • Intracellular fluid (ICF)
    • Interstitial
    • Intravascular (plasma)
  • Extracellular fluid (ECF)
    • Transcellular
    • Lymph

Calculation of Fluid Gain or loss

  • One liter of water weighs 2.2 lbs. (1 kg)
  • Body weight change is an excellent indicator of overall fluid volume loss or gain


  • Substances whose molecules dissociate into ions when placed in water
    • Cations: positively charged
    • Anions: negatively charged
  • Concentration of electrolytes is expressed in milliequivalents (mEq)/L
  • ICF
    • Prevalent cation is K+
    • Prevalent anion is PO43-
  • ECF
    • Prevalent cation is Na+
    • Prevalent anion is Cl-