Consider Ficks Law of Diffusion; the rate of diffusion is proportional to concentration and surface area.
The membrane surface area is vast.
The concentration of water in water is also huge (55.5 moles per liter, one mole of water weighting about 18 grams.)
Thus, even with very poor lipid solubility, water is able to diffuse through the membrane, because there is so much water and there is so much membrane.
Only the following membranes are watertight:
The intracellular fluid never really communicates with the outside world.
The ECF is what receives solute and solvent boluses.
Thus, the ECF is really what determines the osmolality of the body fluid.
When the ECF osmolality changes, so does the ICF.
Because one needs to maintain electrical neutrality, sodium is opposed by an equal anion charge.
Sodium and the anions account for 86% of the osmolality of the extracellular fluid.
Sodium is the solute which is under the greatest amount of homeostatic control.
Thus, it can be said that SODIUM CONCENTRATION DETERMINES EXTRACELLULAR FLUID VOLUME.