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Physiology
Concept Version 7
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Potassium Balance Regulation

Potassium is mainly an intracellular ion.

Learning Objective

  • Describe mechanisms of potassium balance regulation


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 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 could excrete the extra potassium.
    • A high plasma potassium increases aldosterone secretion and this increases the potassium loss from the body, restoring balance.

Terms

  • acidosis

    Acidosis is an increased acidity in the blood and other body tissue (i.e., an increased hydrogen ion concentration). If not further qualified, it usually refers to acidity of the blood plasma.

  • alkalotic

    Alkalosis refers to a condition reducing hydrogen ion concentration of arterial blood plasma (alkalemia). Generally, alkalosis is said to occur when pH of the blood exceeds 7.45.

  • Potassium

    Potassium is a chemical element with symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.


Full Text

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 and with an extracellular value of about 13 litres, 52 mmol (ie. less than 1.5%) is present here and only 12 mmol 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. 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.

If there is a high potassium intake, eg. 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 (ie. alkalosis >7.4) favors movement of K+ into the cells whilst a low pH (ie. acidosis) causes movement out of the cell. A high plasma potassium increases aldosterone secretion and this increases the potassium loss from the body, restoring 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 normal total body potassium.

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