Chloride is the major anion found in the fluid outside of cells and in blood. An anion is the negatively charged part of certain substances such as table salt (NaCl) when dissolved in liquid. Sea water has almost the same concentration of chloride ion as human fluids.
The balance of chloride ion (Cl-) is closely regulated by the body. Significant increases or decreases in chloride can have deleterious or even fatal consequences.