L J Appel, Johns Hopkins University, Baltimore, MD,
© 2005 Elsevier Ltd. All rights reserved.
The major intracellular cation in the body is potassium, which is maintained at a concentration of approximately 145mmol/l of intracellular fluid but at much lower concentrations in the plasma and interstitial fluid (3.8-5 mmol/l of extracellular fluid). The high intracellular concentration of potassium is maintained via the activity of the Na+/K+-ATPase pump. Because this enzyme is stimulated by insulin, alterations in the plasma concentration of insulin can affect cellular influx of potassium and thus plasma concentration of potassium. Relatively small changes in the concentration of extracellular potassium greatly affect the extracellular/intracellular potassium ratio and thereby affect nerve transmission, muscle contraction, and vascular tone.
In unprocessed foods, potassium occurs mainly in association with bicarbonate-generating precursors such as citrate and, to a lesser extent, with phosphate. In processed foods to which potassium is added and in supplements, the form of potassium is potassium chloride. In healthy people, approximately 85% of dietary potassium is absorbed. Most potassium (approximately 77-90%) is excreted in urine,
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whereas the remainder is excreted mainly in feces, with much smaller amounts excreted in sweat. Because most potassium that is filtered by the glomer-ulus of the kidney is reabsorbed (70-80%) in the proximal tubule, only a small amount of filtered potassium reaches the distal tubule. The majority of potassium in urine results from secretion of potassium into the cortical collecting duct, a secretion regulated by a number of factors including the hormone aldosterone. An elevated plasma concentration of potassium stimulates the adrenal cortex to release aldosterone, which in turn increases secretion of potassium in the cortical collecting duct.
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