Let’s apply what we have learnt above to the situation where a person drinks 1.5 litres of water.

The osmolarity of bodily fluids is approximately 0.3 OsM (or 300 mOsM).  The total fluid volume of a normal person is about 45 litres, of which 30 litres is inside cells (intracellular volume) and 15 litres is outside the cells (extracellular volume ,which includes plasma).   We can use these figures to determine the total number of solutes present in each compartment.

Osmolarity (osmoles/L H2O) × volume (L) = total solutes (osmoles)

Below is a table of the osmolarities associated with the intracellular and extracellular compartments before taking the drink:

Before drinking Volume osmolarity Total solutes in compartment
Total body 45 litres 0.3 OsM

intracellular 30 litres 0.3 OsM

extracellular 15 litres 0.3 OsM

After drinking 1.5 litres of water, the water will be absorbed by the gastrointestinal tract and will enter the plasma and extracellular fluid and will equilibrate with the intracellular compartment.  As the solutes that are present inside the cell cannot leave the cell, equilibration will only involve the movement or diffusion of water (Osmosis).

Let’s look at what happens to the whole body first:

After drinking Total solutes volume osmolarity
Total body 13.5 osmoles 46.5 litres

The new osmolarity is 0.29 and not 0.3 OsM.  Water will move to equilibrate the intra- and extra- cellular compartments to this NEW osmolarity. To determine the movement of water we work back from the osmolarity.

After drinking Total solutes volume osmolarity
intracellular 9.0 osmoles

0.29
extracellular 4.5 osmoles

0.29

We can see that because the solutes do not move out of the cells, the 1.5 litres of water will distribute so that 1 litre stays in the extracellular space and 0.5 litre diffuses across the cell membranes to the intracellular compartment.

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