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> Here's a good simple analogy. The horse's intercellular fluid is is of a
> certain mixture. Part electrolytes, and part water. Picture it like a
> pitcher of Kool-Aid. If you pour out "Kool-Aid" and add more water it's
> no longer the correct balance...it's not Kool-Aid any more.  If the horse
> needs that particular balance to function, and can only take in more
> water, he's doing more damage than good because he is diluting his
> cellular fluid...it's no longer the right mix. Having a horse that drinks
> tons could get you in trouble if he had no time to eat enough or was not
> given enough electrolytes to balance it out. 
This sort of oversimplification is unfortunately one reason why people
fail to understand electrolytes.  The horse (or any other critter)
isn't just some big homogenous soup in which e-lytes have to remain a
particular concentration.  Each "compartment" has its own particular
"blend" and must maintain that balance--and unbalancing another
compartment can have an adverse effect.

As Joe (or somebody) pointed out, when you dump e-lytes into the
stomach, the stomach then has to pull fluids out of somewhere else to
dilute the e-lytes before they can be absorbed.  As in Barbara's
scenario, this in and of itself can have a drastic effect on an
already-dehydrated horse.  If, on the other hand, you put e-lytes in
other "stuff" (which may hopefully even include some water) the horse
has a head start on this process--if it isn't in something sloppy, then
if it is in something that has to be chewed, he will at least dilute it
with saliva.

Getting from the stomach to the bloodstream is an active process--and
then getting from the blood into the extracellular spaces and into the
cells requires additional steps.  And blood, extracellular fluid, and
intracellular fluid each have unique e-lyte makeups--they are quite
different from each other.  The body works to maintain this equilibrium
by actively pumping ions across cell membranes.  If this active pumping
did not go on continually, then you would get the sort of homogenous
soup in Angie's description, and the horse would die.

So what happens if you suddenly flood one compartment with a bunch of
extra e-lytes?  Hopefully, the cell membranes can keep up with the
extra work, salvaging the ions the body needs and sorting out those
that it doesn't.  But think of a levy with pumps going to keep the
water level down behind it.  If life is normal, the pumps do their work
just fine, and the water level behind the levy stays down.  But if a big
storm comes along and adds more water than the pumps can handle, you get
a flood behind the levy.  Same sort of thing happens if you overwhelm
the pumps in the cell membranes with more e-lytes than they can handle
at a given time.  They get swamped, and the imbalance spills over into
other fluid spaces.

The reservoir in the hindgut provides a steady-state sort of e-lyte
source from the semi-digested forages there (Bruce's description was
very good).  In many horses, that slurry is a sufficient reserve for
even quite strenuous work--even the seemingly overwhelming numbers that
Gayle Ecker reports are provided in less than one day's forage in most
horses.  But if in a given individual, that is insufficient to keep up
with the outgo caused by exertion, then certainly there needs to be
some sort of replacement.  However, in understanding how the whole
system works, one might want to revisit how it is dosed.  Instead of
administering it straight and by the ounce, one might want to consider
more physiologically appropriate ways of getting it into the
stomach--as already mentioned, in some sort of feed, with a slurry
being particularly appropriate.

Now, I'm going to bed too, and tomorrow I'm gonna go to a ride, where my
horse's limiting factors will be his own lack of conditioning and the
lack of conditioning of the idiot on his back, not a lack of
e-lytes--and I will strive to ride honestly to those conditions, rather
than thinking that e-lytes will somehow make them better...

Heidi


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