Regulation of Potassium Excretion and Potassium Concentration in Extracellular Fluid
Extracellular fluid potassium concentration nor-mally is regulated precisely at about 4.2 mEq/L, seldom rising or falling more than ± 0.3 mEq/L. This precise control is neces- sary because many cell functions are very sensitive to changes in extracellular fluid potassium concentration. For instance, an increase in plasma potassium concentration of only 3 to 4 mEq/L can cause cardiac arrhythmias, and higher concentrations can lead to cardiac arrest or fibrillation.
A special difficulty in regulating extracellular potassium concentration is the fact that over 98 per cent of the total body potassium is contained in the cells and only 2 per cent in the extracellular fluid (Figure 29–1). For a 70-kilogram adult, who has about 28 liters of intracellular fluid (40 per cent of body weight) and 14 liters of extracellular fluid (20 per cent of body weight), about 3920 mil-liequivalents of potassium are inside the cells and only about 59 milliequiva-lents are in the extracellular fluid. Also, the potassium contained in a single meal is often as high as 50 milliequivalents, and the daily intake usually ranges between 50 and 200 mEq/day; therefore, failure to rapidly rid the extracellular fluid of the ingested potassium could cause life-threatening hyperkalemia(increased plasma potassium concentration). Likewise, a small loss of potassium from the extracellular fluid could cause severehypokalemia (low plasma potas-sium concentration) in the absence of rapid and appropriate compensatory responses.
Maintenance of potassium balance depends primarily on excretion by the kidneys because the amount excreted in the feces is only about 5 to 10 per cent of the potassium intake. Thus, the maintenance of normal potassium balance requires the kidneys to adjust their potassium excretion rapidly and precisely to wide variations in intake, as is also true for most other electrolytes.
Control of potassium distribution between the extracellular and intracellular compartments also plays an important role in potassium homeostasis. Because over 98 per cent of the total body potassium is contained in the cells, they can serve as an overflow site for excess extracellular fluid potassium during hyper-kalemia or as a source of potassium during hypokalemia. Thus, redistribution of potassium between the intra- and extracellular fluid compartments pro-vides a first line of defense against changes in extracellular fluid potassium concentration.