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13.1: Electrolytes

Electrolytes are compounds that separate into ions (molecules with a charge) in water. Electrolytes can be separated into two classes:

  • Cations: ions that have a positive charge
  • Anions: ions that have a negative charge

The following table summarizes the major intracellular and extracellular electrolytes by giving their milliequivalents (mEq)/L. Milliequivalents are a measure of charge. Thus, a higher value means that the cation or anion is accounting for more charge.  

Table 13.11: Major intracellular and extracellular electrolytes (mEq/L)1,2
Intracellular Extracellular





Potassium (K+) 150

Phosphate (PO4-) 104

Sodium (Na+) 142

Chloride (Cl-) 103

Magnesium (Mg2+) 40

Proteins 57


Bicarbonate (HCO3-) 27


Sulfate (SO42-) 20


Proteins 16

The following figure graphically shows the major intracellular and extracellular cations (green) and anions (red).

Figure 13.11 Major intracellular and extracellular cations (green) and anions (red)2

Electrolytes and proteins are important in fluid balance. Your body is 60% water by weight. Two-thirds of this water is intracellular, or within cells. One-third of the water is extracellular, or outside of cells. One-fourth of the extracellular fluid is plasma, while the other 3/4 is interstitial (between cells) fluid. Thus, when considering total body water, around 66% is intracellular fluid, 25% is interstitial fluid, and 8% is plasma3,4.

Fluid distribution between the different compartments are shown below.

Figure 13.12 Distribution of fluid in the body.Adapted from

References & Links

  1. Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw's perspectives in nutrition. New York, NY: McGraw-Hill.
  2. Whitney E, Rolfes SR. (2011) Understanding nutrition. Belmont, CA: Wadsworth Cengage Learning.
  3. Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.