Overview of Renal Potassium Excretion
Potassium excretion is determined by the sum of three renal processes: (1) the rate of potassium filtration (GFR multiplied by the plasma potassium concentra-tion), (2) the rate of potassium reabsorption by the tubules, and (3) the rate of potassium secretion by the tubules. The normal rate of potassium filtration is about 756 mEq/day (GFR, 180 L/day multiplied by plasma potassium, 4.2 mEq/L); this rate of filtration is usually relatively constant because of the autoregula-tory mechanisms for GFR discussed previously and the precision with which plasma potassium concentra-tion is regulated. Severe decreases in GFR in certain renal diseases, however, can cause serious potassium accumulation and hyperkalemia.
Figure 29–2 summarizes the tubular handling of potassium under normal conditions. About 65 per cent of the filtered potassium is reabsorbed in the proximal tubule. Another 25 to 30 per cent of the filtered potas-sium is reabsorbed in the loop of Henle, especially in the thick ascending part where potassium is actively co-transported along with sodium and chloride. In both the proximal tubule and the loop of Henle, a rel-atively constant fraction of the filtered potassium load is reabsorbed. Changes in potassium reabsorption in these segments can influence potassium excretion, but most of the day-to-day variation of potassium excre-tion is not due to changes in reabsorption in the prox-imal tubule or loop of Henle.
Most Daily Variation in Potassium Excretion Is Caused by Changes in Potassium Secretion in Distal and Collecting Tubules. The most important sites for regulating potas-sium 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 (31 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 fil-trate, 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 potas-sium in the distal segments of the nephron, and potas-sium 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 potas-sium 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 mech-anisms of potassium secretion and the factors that reg-ulate this process.