13.1 Electrolytes

Electrolytes

Electrolytes are compounds that separate into ions (molecules with a charge) in water. These compounds are also commonly referred to as salts. Electrolytes can be separated into 2 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 electrolytes1,2

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.

You might remember the term “osmosis” from a past science course. You might remember that osmosis has something to do with the movement of water across membranes; into or out of cells. You might further remember that osmosis can be driven by solute concentration (the concentration of dissolved substances). The solute concentration that drives osmosis is commonly called osmolality. Very simply, osmolality is the concentration of a dissolved substance, which tends to affect the movement of water. The electrolytes shown in the diagram above are responsible for osmolality. In Figure 13.11, a higher concentration of ions and proteins in the cell would be osmolality, that would ultimately drive the movement of water into the cell. If the concentration of ions and proteins outside of the cells were greater, you would expect that the osmolality would drive the movement of water out of the cell.

Water balance in our bodies takes place everywhere…in all of our organs…all of our tissues…between our individual cells, in a great complex of interactions ultimately moderated by osmolality. Osmolality can drive water movement into and out of tissues and cells, and osmolality can hold water in a particular place.

Fluid distribution between the different compartments of the body are shown in Figure 13.12.

Figure 13.12 Distribution of fluid in the body3,4.

References & Links

• Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw’s perspectives in nutrition. New York, NY: McGraw-Hill.
• Whitney E, Rolfes SR. (2011) Understanding nutrition. Belmont, CA: Wadsworth Cengage Learning.
• Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
• Adapted from http://www.netterimages.com/image/21248.htm