Potassium

• Potassium is major intracellular cation (sodium is major extracellular) – 90% is exchangeable (70% for sodium)
• Equilibrium potential of -90mV
• Total body = 45mmol/kg (2800 mmol per average male)
  • ICF     90% 150-160 mmol/L
  • ECF    2%    3.5-5.0 mmol/L
  • Bone  8%

• Completely and passively absorbed in upper GI tract
• Excretion: Mainly in collecting ducts. Affected by aldosterone (Collecting duct regulator)
• Associations:
  • Serum K is inversely related to serum pH
  • Serum K decreases by 0.3mEq/L for every 0.1U increase in pH above normal
  • Alkalosis:
    • Correction of an alkalotic pH will produce a rise in serum K
    • Alkalosis (increase pH) shifts K intracellularly, lowering serum K
  • Acidosis
    • Shifts K to intravascular space so increasing the serum K
    • Correction of acidosis will produce a decrease in serum K (may drop precipitously eg correction of DKA)

• Function:
  • Major intracellular ion affecting tonicity
  • Antiport molecule in NA/K transport and control of intracellular volume
  • Involved in neuromuscular excitability.
  • The gradient across cell membranes determines the excitability of nerve and muscle cells including myocardium.
  • Regulation of some intracellular processes (protein/glycogen synthesis, carbohydrate metabolism)
  • Of all the electrolytes, rapid changes in potassium concentration can cause the most immediate life threatening emergencies.

Abnormalities of potassium homeostasis:

• Hyperkalaemia
• Hypokalaemia