Sodium is the major positively charged Cation in ECF. It determines ECF volume. Major intravascular ion to affect osmolality… acute increase or decrease in serum sodium will produce an increase or decrease in serum osmolality.

  • Total body sodium = 60mmol/kg (4000mmol in adult male) – Plasma level around 140mmol/L
    • ECF: 50% (140mmol/L)
    • ICF: 5% (15mmol/L)
    • Bone: 45% (125mmol/L)
  • 70% is exchangeable (90% for K)
  • Strong inward gradient to cells (opposite to K) and Equilibrium potential +60mV
  • Na is reabsorbed in the kidney (Excrete only 0.6%), regulated by renin-angiotensin-aldosterone system, ADH, thirst and the kidney
    • 70% proximal tubule
    • 20% ascending loop
    • 5% distal tubule
    • 3% collecting duct
  • Acute changes in Na will cause osmolality changes with free water shift into and out of the vascular space until osmolality equilibrated.
    • Acute ↓Na will shift fluid into the interstitial space and cause cerebral oedema
    • Acute ↑NA will cause water to enter the vascular space
  • Neurological status must be monitored during correction of low or high Na
  • Replace slowly over 48hrs and avoid over-correction

Abnormalities of sodium homeostasis:

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