According to BRS Physiology book:

excessive NaCl intake will lead to an increase in the osmolarity of the Extracellular Fluid (ECF) compartment, and thus will lead to water shift from the Intracellular Fluid (ICF) compartment to the ECF so the ICF osmolarity increases until the osmolarities equal each other.

But if water shifts from the ICF to the ECF, wouldn't the osmolarity of the ECF decrease also as a result of the decreased concentration of the solutes, and this cycle continues?

  • $\begingroup$ What "decreased concentration of the solutes"? $\endgroup$
    – Mithoron
    Commented Sep 29, 2020 at 19:52
  • $\begingroup$ Water is added so the concentration of the solutes in the fluid will decrease . $\endgroup$
    – Positron12
    Commented Sep 29, 2020 at 19:54
  • $\begingroup$ Well, there's an assumption then both salt and any water or whatever were put into ECF and it caused raising the concentration of NaCl otherwise the effect on ICF could wouldn't be like that. Also what kind of question is that? Obviously these concentrations are changing all the time and you didn't mention anything about water in post, one could eat, say, a spoon of salt.. $\endgroup$
    – Mithoron
    Commented Sep 29, 2020 at 20:13
  • $\begingroup$ According to this picture, as the water moves from the ICF to the ECF, the ICF osmolarity will increase, and the ECF osmolarity will decrease. This will continue until they are equal (this simple picture ignores active transport). Don't know why you are saying "the cycle continues" since, according to this picture, they meet somewhere in the middle, at which point the net flow of water stops. Of course, actual physiology is far more complicated than this. $\endgroup$
    – theorist
    Commented Sep 30, 2020 at 5:25
  • $\begingroup$ If the osmolarity of the ECF decreases, then water should shift from the ECF to the ICF, thus the ICF osmolarity will decrease, so water will shift from the ICF to the ECF, how could this cycle stop? @theorist $\endgroup$
    – Positron12
    Commented Sep 30, 2020 at 7:35

1 Answer 1


If there is a difference in osmolarity and therefore in the osmotic pressure, water flows from solution with lower osmotic pressure to the higher one, decreasing this difference and therefore the flow itself as well.

The kinetic of this process follows the first order kinetics. The flow rate is proportional to the osmotic pressure difference. Therefore, the water flow exponentially decreases with time, as the osmotic pressure difference decreases, reaching the equilibrium theoretically ( mathematically ) in infinite time.

$$ \frac {\mathrm{d}\Delta p}{\mathrm{d}t} = - k \cdot \Delta p$$

$$ \Delta p = \Delta p_{t=0} \cdot \exp {(-k \cdot t) }$$

The kinetic constant $k$ depends on the membrane permeability, water diffusion coefficient and geometry of the scenario.

Therefre, there is no cyclic oscillation of osmotic pressures like:

  • 1 - 9 -> 9 - 1 -> 1 - 9

but there is an exponential convergence like

  • 1 - 9 -> 3 - 7 -> 4 - 6 -> 5 - 5

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