In article , Neil wrote:

Fluid balance is an art form as well as a science. A typical adult in
the UK needs about 90 millimole of sodium per day and about 80 of
potassium and enough water to replace what goes out as urine plus
sweat losses (anything from 500mL to 5000 mL perday depending on the
situation).

The problem is solutions that are not "normal" ie same osmotic
strength as cells tend to cause blood problems. ( You can show this to
children using raw potato cut into chips all the same length put one
into pure water and one into a strong salt solution and the water one
swells and lenghtens the one in saline shrinks and becomes floppy. At
0,9% no change).

The upshot of this is set strength tend to be used. Glucose is
metabolised off so it provides water the glucose provides the right
concentration without leaving a residue so to speak. You are quite
right potassium is needed but in the short term ( 24 hrs) you can do
without. After that it is added typically 20 -40 millimole per litre
of fluid.

My understanding is that normal saline as used in hospitals contains
potassium as well as sodium, and perhaps magnesium and calcium as
well, because it is more general and safer.

For example, if someone is admitted suffering from dehydration and
is actually short of potassium, it is NOT good to rehydrate them with
a potassium-free saline! You need a blood analysis to be certain,
and there often isn't time.


Regards,
Nick Maclaren.