Electrolytes are substances that dissociate into electrically charged ions when dissolved. Cations carry positive charges and anions carry negative charges. Both affect the electrical and osmolal (i.e., the number of particles dissolved in a fluid) functioning of the body. Body fluids always contain equal numbers of positive and negative charges, but the nature of the ions, the number of charges present on a single molecule, and the nature and mobility of the charged molecules differ enormously among body fluid compartments (e.g., intracellular versus extracellular).
Not all charged particles are ions. Proteins, for example, carry a net negative charge. Whenever fluid contains protein, there must be accompanying cations. Similarly, not all solutes found in plasma are ions. Urea and glucose, for example, do not dissociate; they do not contribute to electrical activity of fluids and membranes, and they contribute only moderately to plasma osmolality.
Electrolyte quantities and the balance among them in the body fluid compartments are controlled by (1) oxygen and carbon dioxide exchange in the lungs; (2) absorption, secretion, and excretion of many substances by the kidneys; and (3) secretion of regulatory hormones by the endocrine glands.
Quantitatively, the most important body fluid ions are sodium, potassium, chloride, and bicarbonate. These ions are measured in routine serum electrolyte determinations. Other serum ions that may be measured include calcium, magnesium, and phosphorus.60