User Tools

Site Tools


potassium

potassium physiology

potassium physiology and homeostasis

total body potassium

  • total body potassium
    • = 45 meq/kg (young adult males as mainly prop.to muscle mass);
    • = 20 meq/kg (malnourished, wasted person);
  • non-exchangeable = 10% - bone 7.6%;
  • exchangeable = 41 meq/kg (90%) - intracellular (89.6%), plasma (0.4%);
  • ie. for average adult, total body K = 250g with 1g in plasma & average daily intake of 10g which is normally excreted renally within the limits of 1-40g/day but in pts with renal disease or on K-sparing diuretics, the renal excretion may be restricted to 5g/day, hence potentially allowing for build up in total body potassium and thus hyperkalaemia.

regulation:

  • end-point for homeostatic control of [K] is:
    • ratio [K]i : [K]e to maintain transmembrane potential;
    • constant [K]i important for many i/cellular functions;

potassium intake:

  • K is present at ~constant levels in animal and vegetable tissues av. 300mg/100g, but processed foods vary 10-4000mg/100g;
  • No regulation;
  • normal intake 1-2mEq/kg/d but varies

GIT absorption:

  • reasonably complete in upper GIT but body K exchanged for lumenal Na in lower GIT;

renal excretion:

  • 90% filtered K is reabsorbed in prox. tubule along with water & thus little is presented to thick ascending limb with the final urine [K] dependent on K secretion in distal tubule & water reabsorption:
    • increased distal tubule secretion due to:
      • increased Na distal tubule reabsorption ( increased by increased serum [K])
      • aldosterone (partly due to pre-existing K intake levels)
      • increased tubular fluid flow rate (eg. diuretics) → incr. [] gradient
  • daily excretion of potassium:
    • normally 50-100mmol;
    • stressed 10-500mmol;
    • renal disease or K-sparing diuretics 10-60mmol;

translocation or redistribution of potassium into or out of cells:

  • important way of dealing with acute K load is to translocate into cells, esp. liver, muscle, which is increased by:
    • beta 2 adrenergic receptor activation (eg. salbutamol)
    • aldosterone - lowers slope of linear relationship of [K]serum vs total body K
    • amino acids (eg. TPN or administered amino acids)
    • periodic hyperkalaemic paralysis & periodic hypokalaemic paralysis
    • direct Na-K ATPase inhibition: digoxin toxicity, suxamethonium
    • alkalosis → incr. cell uptake and vice versa for acidosis
  • ⇒ need to correct serum [K] for pH & exclude other causes of redistribution to ascertain total body K, as “corrected [K]” of 1mM is approx. 6-10% total body K
    • for each decrease in pH by 0.1 ⇒ se [K] increases by ~0.7 (artificially high)
    • for each increase in pH by 0.1 ⇒ se [K] decreases by ~0.6 (artificially low)
    • thus, if serum [HCO3] abnormal, then pH likely to be abnormal → need to calculate adjusted [K] to allow for redistribution before deciding to treat the [K] level.
potassium.txt · Last modified: 2014/12/15 14:16 (external edit)