Water movement between the ICF and ECF compartments is determined by:

We have electrolytes in our blood, urine (pee), tissues, and other body fluids. Electrolytes are important because they help: Balance the amount of water in our body. Balance our body's acid/base (pH) level.
Common electrolyte imbalances include hyponatremia, hypokalemia, hyperkalemia, hypocalcemia, hypochloremia, and hypophosphatemia. Acid-base imbalances, either acidemia or alkalemia, occur as a result of the addition of acid and depletion of alkali reserve, or the loss of acid with a relative increase in alkali reserve.