The osmolarity of a solution is inversely proportional to the water concentration. Water tends to flow from an area of lower osmolarity to an area of higher osmolarity. Until the osmolarity of both is equal. If the osmolarity of the ECF is changed say after a meal high in salt then because the ECF now has a higher osmolarity than the interstitial fluid water moves from the interstitium into the ECF until a neq equilibrium is reached. Water in turn flows out of the intracellular fluid into the interstitium as this now has a higher osmolarity. Thus cell volume changes.