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25.3C: Potassium Balance Regulation

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    8180
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    Potassium is mainly an intracellular ion.

     

    LEARNING OBJECTIVES

     

    Describe the mechanisms of potassium balance regulation

     

    KEY TAKEAWAYS

    Key Points

     

    • Most of the total body potassium is inside the cells and the next largest proportion is in the bones.
    • In an unprocessed diet, potassium is much more plentiful than sodium and it is present as an organic salt, while sodium is added as NaCl.
    • High potassium intake can potentially increase the extracellular K+ level two times before the kidney can excrete the extra potassium.
    • A high plasma potassium increases aldosterone secretion and this increases the potassium loss from the body to restore balance.

     

    Key Terms

     

    • alkalotic: A condition that reduces the hydrogen ion concentration of arterial blood plasma (alkalemia). Generally, alkalosis is said to occur when the blood pH exceeds 7.45.
    • Potassium: A chemical element with the symbol K and the atomic number 19. Elemental potassium is a soft, silvery white, alkali metal that oxidizes rapidly in the air and is very reactive with water—it can generate sufficient heat to ignite the hydrogen emitted in the reaction.
    • acidosis: An increase in acidity of the blood and other body tissue (i.e., an increased hydrogen ion concentration). If not further qualified, it usually refers to the acidity of the blood plasma.

     

    Potassium Balance

     

    Potassium is predominantly an intracellular ion. Most of the total body potassium of about 4,000 mmol is inside the cells, and the next largest proportion (300–500 mmol) is in the bones. Cell K+ concentration is about 150 mmol/l but varies in different organs. Extracellular potassium is about 4.0 mmol/l, with an extracellular value of about 13 liters, 52 mmol (i.e., less than 1.5%) is present here and only 12 mmol is in the plasma.

    In an unprocessed diet potassium is much more plentiful than sodium. It is present as an organic salt, while sodium is added as NaCl. In a hunter-gatherer, K+ intake may be as much as 400 mmol/d while in the Western diet it is 70 mmol/d or less if a person has a minimal amount of fresh fruit and vegetables. 

    The processing of foods replaces K+ with NaCl. While the body can excrete a large K+ load, it is unable to conserve K+. On a zero K+ intake, or in a person with K+ depletion, there will still be a loss of K+ of 30–50 mmol/d in the urine and feces.

     

    Acid– Base Status Control

     

    If there is a high potassium intake, for example, 100 mmol, this would potentially increase the extracellular K+ level two times before the kidney could excrete the extra potassium. The body buffers the extra potassium by equilibrating it within the cells. 

    The acid–base status controls the distribution between plasma and cells. A high pH (i.e., alkalosis >7.4) favors the movement of K+ into the cells, and a low pH (i.e., acidosis ) causes movement out of the cell. A high plasma potassium level increases aldosterone secretion and this increases the potassium loss from the body to restore balance. 

    This change of distribution with the acid–base status means that the plasma K+ may not reflect the total body content. Therefore, a person with an acidosis (pH 7.1) and a plasma K+ of 6.5 mmol/l could be depleted of total body potassium. This occurs in diabetic acidosis. Conversely, a person who is alkalotic with a plasma K+ of 3.4 mmol/l may have a normal level of total body potassium.