Movement of water between intracellular and extracellular body fluid compartments

How does water traverse the supposedly water-impermeable lipid bilayer?

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.

Water is easily able to move across most cell membranes.

Water will move between compartments until their osmolality is the same

Only the following membranes are watertight:

      • Bladder epithelium
      • Ascending limb of the Loop of Henle
      • Cortical collecting duct in absence of vasopressin

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.

What is responsible for the osmolality of the extracellular fluid? Which solute?

Its sodium.

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.

References

anaesthesiamcq.org, as always; my source for most of this information has been the Kerry Brandis Anaesthetic Viva book.  Everyone should have one.

Schoeller DA, van Santen E, Peterson DW, Dietz W, Jaspan J, Klein PD: Total body water measurement in humans with 18O and 2H labeled water. Am J Clin Nutr 1980, 33(12):2686-2693.