Definition of Sodium

Sodium: The major positive ion (cation) in fluid outside of cells. The chemical notation for sodium is Na⁺. When combined with chloride, the resulting substance is table salt.

Excess sodium (such as from fast food hamburger and fries) is excreted in the urine. Too much or too little sodium can cause cells to malfunction, and extremes can be fatal.

Normal blood sodium level is 135 - 145 milliEquivalents/liter (mEq/L), or in international units, 135 - 145 millimoles/liter (mmol/L).