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Potassium is the major intracellular cation (intracellular K+ concentration is approximately 140 mEq/L) and is important for maintaining resting membrane potential of cells. 60-75% of total body potassium is found within muscle cells, with the remainder in bone. Only 5% of potassium is located in the ECF, therefore potassium concentration in blood is not always a reflection of total body potassium levels. Plasma (ECF) K+ concentration is tightly regulated; fairly small changes can have marked effects on organ function. Regulation of potassium Ingested K+ is absorbed non-selectively in the stomach and small intestine. Regulation of plasma K+ is by renal excretion and movement of K+ from extracellular fluid to intracellular fluid. If these mechanisms are functioning normally, the amount of K+ ingested has little affect on plasma K+. However, if one or more of the regulatory mechanisms is faulty, then the amount of K+ ingested can exacerbate abnormalities in plasma K+. Urinary excretion of K+ is largely by secretion of K+ into the urine by the distal tubules. 70% of filtered K+ is absorbed in the PCT of the kidney regardless of K+ balance. 20% is absorbed in the ascending limb of the loop of Henle and the remaining 10% is delivered to the distal nephron. Principle cells in the distal nephron secrete K+ and absorb Na+ under the influence of aldosterone. Intercalated cells in the distal nephron absorb K+ in exchange for H+. Secretion of K+ by the distal nephron is governed by: the extracellular concentration of K+, aldosterone, distal tubule flow rate (increases in flow rate enhances K+ secretion), lumen electronegativity (increased electronegativity enhances secretion), concentration of NaCl in the distal tubule lumen (low Na+ decreases secretion whilst low Cl enhances secretion) and ADH (stimulates secretion). K+ is also excreted in the colon, which is also influenced by aldosterone. Translocation of K+ is largely dependent on insulin and catecholamines. Shifts of K+ in and out of cells can also occur with changes in the pH of ECF. Severe abnormalities of plasma K+ are life-threatening situations. [Hyperkalemia] 
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