Fluid Balance

Excerpt from the wiki on Fluid Balance:

Fluid balance is an aspect of the homeostasis of living organisms in which the amount of water in the organism needs to be controlled, via osmoregulation and behavior, such that the concentrations of electrolytes (salts in solution) in the various body fluids are kept within healthy ranges.

Homeostasis: "Homeostasis or homoeostasis (homeo- + -stasis) is the property of a system in which variables are regulated so that internal conditions remain stable and relatively constant."

Its important to note that it is electrolyteS (plural) and the concentrations of each type of electrolytes effect the regulatory reactions proportionately.

Gatorade has certain amounts of BOTH sodium and potassium in it as these are TWO important electrolytes for the human body that are lost via perspiration.  The concentrations of EITHER of these two individually are important to CORRECT function.  ie If you are low on sodium, you cant just take in potassium or vice versa... BUT, the TOTAL electrolyte concentration is also influential to water flow.