Electrolytes are molecules, classified as salts, which are distributed in the fluids in the body. These salts are ionic; the sum of all the charged atomic constituents (electrons and protons) is not zero, but is either a positive charge or a negative charge. Positively charged ions are called cations and negatively charged ions are called anions. The most prominent examples of electrolytes found in the human body are potassium, sodium, calcium, and chloride.

Body fluids such as blood, plasma and the so-called interstitial fluid that flows between cells are much like seawater in composition, having a high concentration of sodium chloride (NaCl). The cation in NaCl is Na⁺ and the anion is Cl^-.

In a given compartment, the number of positively charged molecules will be balanced by the number of negatively charged molecules, so that the fluid will be electrically neutral...