Regulation

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A. Shifts of K+ between the ICF and ECF (Table 5-4)

- A shift of K+ out of cells causes hyperkalemia.

- A shift of K+ into cells causes hypokalemia.

B. Renal regulation of K+ balance (Figure 5-11)

- K+ is filtered, reabsorbed, and secreted by the nephron.

- K+ balance is achieved when urinary excretion of K+ exactly equals intake of K+ in the diet.

- K+ excretion can vary widely from 1% to 110% of the filtered load, depending on dietary K+ intake, aldosterone levels, and acid-base status.

Table 5-4. Shifts of K+ Between ECF and ICF

Causes of Shift of K+ out of Cells -> Hyperkalemia

Causes of Shift of K+ into Cells —> Hypokalemia

Insulin deficiency

Insulin fj-Adrenergic antagonists

(3-Adrenergic agonists

Acidosis (exchange of extracellular H+ for intracellular K+)

Alkalosis (exchange of intracellular H+ for extracellular K+)

Hyperosmolarity (H20 flows out of the cell; K+ diffuses out with H20)

Hyposmolarity (H20 flows into the cell; K* diffuses in with H20)

Inhibitors of Na+-K+ pump (e.g., digitalis) [when pump is blocked, K+ is not taken up into cells]

Exercise

Cell lysis

ECF = extracellular fluid; ICF = intracellular fluid.

Figure 5-11. K* handling along the nephron. Arrows indicate reabsorption or secretion of K\ Numbers indicate the percentage of the filtered load of K+ that is reabsorbed, secreted, or excreted.

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