Chapter 046. Sodium and Water (Part 11)

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Chapter 046. Sodium and Water (Part 11) Chapter 046. Sodium and Water (Part 11) Potassium: Introduction Potassium Balance Potassium is the major intracellular cation. The normal plasma K+concentration is 3.5–5.0 mmol/L, whereas that inside cells is about 150 mmol/L.Therefore, the amount of K+ in the ECF (30–70 mmol) constitutes and inhibited in the setting of digoxin toxicity or chronic illness such as heartfailure or renal failure. The K+ intake of individuals on an average western diet is 40–120 mmol/d,or approximately 1 mmol/kg per day, 90% of which is absorbed by thegastrointestinal tract. Maintenance of the steady state necessitates matching K +ingestion with excretion. Initially, extrarenal adaptive mechanisms, followed laterby urinary excretion, prevent a doubling of the plasma K + concentration thatwould occur if the dietary K+ load remained in the ECF compartment.Immediately following a meal, most of the absorbed K+ enters cells as a result ofthe initial elevation in the plasma K+ concentration and facilitated by insulinrelease and basal catecholamine levels. Eventually, however, the excess K + isexcreted in the urine (see below). The regulation of gastrointestinal K + handling isnot well understood. The amount of K+ lost in the stool can increase from 10 to50% or 60% (of dietary intake) in chronic renal insufficiency. In addition, colonicsecretion of K+ is stimulated in patients with large volumes of diarrhea, resultingin potentially severe K+ depletion. Potassium Excretion (See also Chap. 272) Renal excretion is the major route of elimination ofdietary and other sources of excess K+. The filtered load of K+ (GFR x plasma K+concentration = 180 L/d x 4 mmol/L = 720 mmol/d) is ten- to twentyfold greaterthan the ECF K+ content. Some 90% of filtered K+ is reabsorbed by the proximalconvoluted tubule and loop of Henle. Proximally, K + is reabsorbed passively withNa+ and water, whereas the luminal Na+-K+-2Cl– co-transporter mediates K+uptake in the thick ascending limb of the loop of Henle. Therefore, K+ delivery tothe distal nephron [distal convoluted tubule and cortical collecting duct (CCD)]approximates dietary intake. Net distal K+ secretion or reabsorption occurs in thesetting of K+ excess or depletion, respectively. The cell responsible for K+ secretion in the late distal convoluted tubule (orconnecting tubule) and CCD is the principal cell. Virtually all regulation of renalK+ excretion and total body K+ balance occurs in the distal nephron. Potassium secretion is regulated by two physiologic stimuli—aldosteroneand hyperkalemia. Aldosterone is secreted by the zona glomerulosa cells of theadrenal cortex in response to high renin and angiotensin II or hyperkalemia. The plasma K+ concentration, independent of aldosterone, can directlyaffect K+ secretion. In addition to the K+ concentration in the lumen of the CCD,renal K+ loss depends on the urine flow rate, a function of daily solute excretion(see above). Since excretion is equal to the product of concentration and volume,increased distal flow rate can significantly enhance urinary K + output. Finally, insevere K+ depletion, secretion of K+ is reduced and reabsorption in the cortical andmedullary collecting ducts is upregulated. Hypokalemia Etiology (Table 46-3) Hypokalemia, defined as a plasma K+ concentration