Clinical Potassium Physiology

Potassium Homeostasis = balancing "ins and outs"

most abundant monovalent cation in body.  concentration in intracellular space is about the same as that of Na in extracellular space (140 mEq/L).  but the ICF volume is 2x ECF volume so there is twice as much K.

Em sensitive to extracellular K due to relatively small concentrations of K.  Em not sensitive to same magnitude of change in intracellular fluid. 




ECF has a total of 70 mEq of K.  A meal may contain 70 mEq K.  Portal vein receives K load, insulin is released, and insulin moves K into cells where the total K is 3500 mEq K.  Then K slowly leaches out into blood to be excreted in urine.




Em is balance of concentration gradient (in to out) and electrical gradient (out to in).


Factors that move K into cells:

• Insulin drives K into cells by stimulating Na-K ATPase
• beta 2 stimulation - stimulate Na-K ATPase
• alkalosis -  shift of H+ and K+ to maintain electrical neutrality
• anabolic activity

Factors that move K out of cells:

• acidosis
• more in mineral acidosis
• mile in organic acidosis (e.g., lactic; ketoacidosis) - anion moves into cells so no need for shift to maintain electrical neutrality
• increased tonicity; e.g., diabetics.  osmotic shift of H2O our of cells and pulls K with H2O
• beta blockers
• alpha stimulation 
• exercise (due to muscle repolarization) also decrease in ATP in exercising muscles opens K channels in membrane allowing K to leak out.  Increase of K in ECF contributes to vasodilation of VSM and is adaptive to exercise.

Internal K balance

 Hypokalemia

 Hyperkalemia

Renal Handling of Na and K+

Na+ 25,000 mEq per day are filtered and most is reabsorbed.  < 1% of that is excreted in urine.  Total body Na+ determines blood volume (amount of Na+  NOT Na+ concentration)

K+ 720 mEq per day are filtered.  All of this is reabsorbed.  Then K is secreted in collecting ducts.  Regulated by aldosterone levels and Na delivery to collecting duct.  Increased aldo stimulates Na reabsorption and increase K excretion into urine.

Na and K balance = delivery to distal tubule balanced by aldosterone

Causes of Hypokalemia 

• shifts into cells - acute but not chronic
• decreased intake
• renal loss - 
• excess aldo
• renal artery stenosis
• renin secreting tumor
• aldo secreting adenoma (Conn's tumor)
• congenital adrenal hyperplasia
• Excess cortisol- acts on aldo receptor
• Cushing syndrome
• genetic defects in enzymes that break down cortisol (hydroxylase)