Most Daily Variation in Potassium Excretion Is Caused by Changes in Potassium Secretion in Distal and Collecting

Tubules. The most important sites for regulating potassium excretion are the principal cells of the late distal tubules and cortical collecting tubules. In these tubular segments, potassium can at times be reabsorbed or at other times be secreted, depending on the needs of the body.With a normal potassium intake of 100 mEq/day, the kidneys must excrete about 92 mEq/day (the remaining 8 milliequivalents are lost in the feces). About one third (3l mEq/day) of this amount of potassium is secreted into the distal and collecting tubules.

With high potassium intakes, the required extra excretion of potassium is achieved almost entirely by increasing the secretion of potassium into the distal and collecting tubules. In fact, with extremely high potassium diets, the rate of potassium excretion can exceed the amount of potassium in the glomerular filtrate, indicating a powerful mechanism for secreting potassium.

When potassium intake is reduced below normal, the secretion rate of potassium in the distal and collecting tubules decreases, causing a reduction in urinary potassium secretion. With extreme reductions in potassium intake, there is net reabsorption of potassium in the distal segments of the nephron, and potassium excretion can fall to 1 per cent of the potassium in the glomerular filtrate (to less than 10 mEq/day). With potassium intakes below this level, severe hypokalemia can develop.

Thus, most of the day-to-day regulation of potassium excretion occurs in the late distal and cortical collecting tubules, where potassium can be either reabsorbed or secreted, depending on the needs of the body. In the next section, we consider the basic mechanisms of potassium secretion and the factors that regulate this process.

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Essentials of Human Physiology

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