Well over half of the body's weight is made up of water. Doctors think about the body's water as being restricted to various spaces, called fluid compartments. The three main compartments are
- Fluid within cells
- Fluid in the space around cells
To function normally, the body must keep fluid levels from varying too much in these areas.
Some minerals—especially the macrominerals (minerals the body needs in relatively large amounts)—are important as electrolytes. Electrolytes are minerals that carry an electric charge when they are dissolved in a liquid such as blood. The blood electrolytes—sodium, potassium, chloride, and bicarbonate—help regulate nerve and muscle function and maintain acid-base balance and water balance.
Electrolytes, particularly sodium, help the body maintain normal fluid levels in the fluid compartments because the amount of fluid a compartment contains depends on the amount (concentration) of electrolytes in it. If the electrolyte concentration is high, fluid moves into that compartment (a process called osmosis). Likewise, if the electrolyte concentration is low, fluid moves out of that compartment. To adjust fluid levels, the body can actively move electrolytes in or out of cells. Thus, having electrolytes in the right concentrations (called electrolyte balance) is important in maintaining fluid balance among the compartments.
The kidneys help maintain electrolyte concentrations by filtering electrolytes and water from blood, returning some to the blood, and excreting any excess into the urine. Thus, the kidneys help maintain a balance between daily consumption and excretion of electrolytes and water.
If the balance of electrolytes is disturbed, disorders can develop. For example, an electrolyte imbalance can result from the following: