Absorption and excretion of potassium through the kidneys

Renal excretion of potassium is determined by the sum of three processes: (1) potassium filtration rate (glomerular filtration rate [GFR] times plasma potassium concentration), (2) rate of potassium reabsorption in the renal tubules, and (3) the rate of potassium excretion by the renal tubules. The normal rate of potassium filtered through glomerular capillaries is about 756 mEq/day (GFR, 180 L / day multiplied by plasma potassium concentration, 4.2 mEq / L). This filtration rate is relatively stable in healthy subjects due to the previously discussed auto-regulation mechanisms for GFR and the precision with which plasma potassium concentrations are adjusted. However, a severe drop in GFR in some kidney diseases can cause severe accumulation of potassium and hyperkalaemia. A summary of the potassium treatment under normal conditions. About 65 percent of the filtered potassium is reabsorbed in the proximal tubule. Another 25 to 30 percent of the filtered potassium is reabsorbed in the Henle loop, especially in the thick part where potassium is actively co-transported with sodium and chloride. In both the prox tubule and the Henle loop, a relatively constant portion of the filtered potassium is reabsorbed. Changes in potassium reabsorption in these segments can affect potassium excretion, but most of the daily variability in potassium excretion is not due to changes in proximal tubular reabsorption. or the Henle strap.

There is also some potassium reabsorbed in the manifold; The amount of reabsorption in these parts of the nephron varies with the amount of potassium ingested.

Figure. Location of renal tubular reabsorption and excretion of potassium. Potassium is reabsorbed in the proximal tubule and in the ascending loop of Henle, so only about 8 percent of the filtered load is delivered to the distal tubule. The excretion of potassium by the main cells of the distal tubule and the manifold increases the amount to be delivered, but some alternate cells are further reabsorbed; therefore, the daily amount of potassium excreted is about 12% of the filtered potassium in the glomerular capillaries. The percentage indicates the amount of filtered load that is reabsorbed or excreted into different tube segments.

Daily changes in potassium excretion are caused mainly by changes in potassium excretion in the distal tubules and the manifold. The most important sites for the regulation of potassium secretion are the main cells of the distal tubule and the sheath. In these tubes, potassium can be reabsorbed or sometimes secreted, depending on the needs of the body. With a normal amount of potassium of 100 mEq/day, the kidneys must excrete about 92 mEq/day (the remaining 8 mEq is lost in the faeces).

About 60 mEq/day potassium is secreted into the distal tubule and the manifold, which accounts for the majority of potassium excretion.

With high potassium, potassium excretion is required almost entirely by increasing the excretion of potassium into the distal tubules and the manifold. In fact, in people with too high a potassium diet, the rate of potassium excretion may exceed the amount of potassium in the glomerular filtrate, suggesting a strong mechanism of potassium secretion.

Figure. Mechanism of potassium excretion and sodium reabsorption by the distal tubule and the manifold main cells.

BK, the "large" potassium channel; ENaC, epithelial sodium channel; ROMK, extra-medullary potassium channel.

When potassium is low, potassium excretion in the distal tubule and the manifold decreases, causing decreased urinary potassium excretion. There is also an increase in potassium reabsorption by alternating cells in the distal nephron segments, and potassium excretion may be reduced to less than 1% of the potassium in the glomerular dialysis solution (to <10 mEq/day). When absorbed potassium is below this level, severe hypokalaemia can develop.

Therefore, most of the mechanisms that regulate daily potassium secretion occur at the end of the distal tubule and the manifold, where potassium can be reabsorbed or secreted, depending on the body's needs.