POTASSIUM

Potassium: serum and urinary.

Serum potassium

Potassium is the main electrolyte (cation) and constituent of the intracellular fluid buffer system. 90% of the potassium is concentrated inside the cell, only small amounts being present in the bones and blood. The damaged cells release potassium into the blood. The entire amount of potassium contained in ingested food is absorbed in the small intestine. The vast majority of potassium (90%) is in ionic form, the rest being protein-related. 

Potassium is indispensable for the normal development of membrane electrical phenomena. It also plays an important role in nerve conduction, muscle contraction, acid-base balance, osmotic pressure, protein anabolism and glycogen formation. Anabolic processes are accompanied by the fixation of potassium in the cell, and catabolic processes by its release. 

Along with calcium and magnesium, potassium controls heart contraction and flow. Potassium and sodium ions are important in the renal regulation of acid-base balance, hydrogen ions being replaced by sodium and potassium ions in the renal tube. Potassium bicarbonate is the major intracellular inorganic buffer. 

Biological reference range:

Premature - umbilical cord blood: 5.0-10.2 mEq/L; premature 48h - venous blood: 3.0-6.0 mEq/L; newborns - umbilical cord blood: 5.6-12.0 mEq/L; newborns - venous blood: 3.7-5.9 mEq/L; children - venous blood: 3.4-4.7 mEq/L; adults: 3.5-5.1 mEq/L. 

Urinary potassium

The greatest amount of potassium is eliminated from the body through the kidneys. Normally 80-90% of potassium is excreted in the urine and the rest in sweat and stool. At the level of the nephron, potassium is subjected to the three fundamental mechanisms: glomerular filtration, complete reabsorption in the proximal tubules and secretion in the distal tubules. 

Biological reference range:

Girls (6-9y): 17-54 mEq/24h; boys (6-9y): 8-37 mEq/24h; girls (10-14y): 22-57 mEq/24h; boys (10-14y): 18-58 mEq/24h; adults: 25-125 mEq/24h.