Disorders of Potassium Balance
Potassium plays a major role in the electrophysiol-ogy of cell membranes as well as in carbohydrate and protein synthesis . The resting cell membrane potential is normally dependent on the ratio of intracellular to extracellular potassium concentrations. Intracellular potassium concentra-tion is estimated to be 140 mEq/L, whereas extra-cellular potassium concentration is normally about 4 mEq/L. Under some conditions, a redistribution of K+ between the ECF and ICF compartments can result in marked changes in extracellular [K +] with-out a change in total body potassium content.
Dietary potassium intake averages 80 mEq/d in adults (range, 40–140 mEq/d). About 70 mEq of that amount is normally excreted in urine, whereas the remaining 10 mEq is lost through the gastrointes-tinal tract.
Renal excretion of potassium can vary from as little as 5 mEq/L to over 100 mEq/L. Nearly all the potassium filtered in glomeruli is normally reab-sorbed in the proximal tubule and the loop of Henle. The potassium excreted in urine is the result of distal tubular secretion. Potassium secretion in the distal tubules is coupled to aldosterone-mediated reab-sorption of sodium .
Extracellular potassium concentration is deter-mined by cell membrane Na+–K+-ATPase activity and plasma [K+], and is influenced by the balance of potassium intake and excretion. Cell membrane Na+–K+-ATPase activity regulates the distribution of potassium between cells and ECF, whereas plasma [K+] is the major determinant of urinary potassium excretion.
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