POTASSIUM HOMEOSTASIS

Potassium: a critical cation

Potassium (K+) is a main cation essential for many cellular functions. It is the most abundant cation in the body, with 98% in intracellular fluid and only 2% in extracellular fluid. Because of this delicate balance, even the slightest acute compartmental shifts can be fatal.1-3

The pathophysiology of serum potassium homeostasis

Regulation of serum potassium is complex and involves appropriate distribution between intracellular and extracellular compartments, and a balance between dietary and supplemental intake and bodily excretion.3,4 (See a list of potassium-rich foods.) The kidney is central to this process, which also relies on several other regulators.4 In healthy individuals, homeostasis is maintained when cellular uptake and urinary or renal excretion naturally counterbalance a patient's dietary intake of K+.4 While values vary from person to person and day to day, the standard normal range of serum potassium is considered to be 3.5 mEq/L to 5.0 mEq/L.5,6

This illustration depicts a human head and torso and lists out data on the potassium homeostasis.

Dietary intake, distribution, and excretion of potassium

In healthy individuals, 90% of K+ is absorbed through the gut into the serum where the majority is taken up by cells.

There is a delicate balance between intracellular and extracellular levels, with nearly 98% of K+ found in the intracellular compartment.7 The ratio of intracellular to extracellular K+, managed primarily by sodium-potassium ATPase, is important in determining the cell membrane potential.7 Other ion channels and transporters can also play a role in affecting this distribution.7

About 70% of the total intracellular content is distributed primarily in the muscles, with smaller amounts present in bone, red blood cells, liver, and skin.3

DISTRIBUTION OF TOTAL BODY POTASSIUM3
This table lists out the distribution of potassium throughout the body.

The kidney plays a major role in the maintenance of K+ balance by adjusting secretion in response to dietary intake, regulating about 90% of excretion. In addition to the kidney's role in regulating serum K+, other processes also affect serum concentration by shifting the intracellular and extracellular balance. For example, acidosis and hyperosmolarity can increase K+ release from cells, while insulin, catecholamines via β2 receptors, and mineralocorticoids can all promote cellular K+ uptake.4

The kidney’s critical role in potassium regulation

In healthy individuals, about 90% of daily ingested K+ is excreted through urine and about 10% is excreted through the colon. The total output remains approximately the same as the total intake. Once K+ is absorbed, it circulates to the kidneys, where it is regulated through a complex process of active secretion and absorption in the distal tubule and collecting ducts.4

Aldosterone plays a key role in K+ elimination via the kidneys by increasing the density and activity of sodium-potassium ATPase pumps in the basolateral membrane. The net result is an increase in K+ permeability and driving force, which promotes K+ secretion.8