ABSTRACT
Sodium is a major extracellular cation that acts as an effective osmole, i.e. it does not cross cell membranes readily, and can exert osmotic effects across a gradient. Water will follow by osmosis across a cellular membrane, depending on the concentration of sodium on either side of the membrane. Water will move from an area of lower sodium (higher water) concentration to higher sodium sodium concentration. In patients with a variety of disease states, hypernatremia and hyponatremia are commonly observed.
