Ion

A particle that carries an electrical charge; positive ions are called cations and negative ions are called anions. Many vital body processes, such as the transmission of nerve impulses, depend on the movement of ions across cell membranes. Sodium is the principal cation in the fluid that bathes all cells (extracellular fluid). It affects the flow of water into and out of cells (see osmosis), thereby influencing the concentration of body fluids. The acidity of blood and other body fluids depends on the level of hydrogen cations, which are produced by metabolic processes. To prevent the fluids from becoming too acidic, hydrogen cations are neutralized by bicarbonate anions in the extracellular fluid and blood, and by phosphate anions inside cells (see acid–base balance).