Re: Am I hypotonic to the pool water when I swim; is this why I am dehydrated?

To: Sarah Joy
ANSWER:
You are actually hypertonic to the pool water when you swim.  Roughly put, 
this means the salinity (a rough measure of total ion content) of your body 
fluids is greater than that of the pool water.  If anything, your body 
would absorb some pool water if there were no active cellular 
osmoregulation (you were dead).

The actual amount of dehydration/hydration depends a lot on where you swim, 
and for how long you swim there.  Basically, you exercise when you swim, 
you sweat when you exercise, sweating causes loss of body fluids, and you 
can't feel yourself sweat in the water.  So regardless of where you 
exercise or the salinity of the water, you do lose some fluids through 
exercise (respiration).  This alone could lead to dehydration.  Factor in 
some urination-a typical response of a human in water (some says this goes 
back to an early survival response of not carrying around any more weight 
than absolutely necessary), and it could easily be measurable.

Your lips and eyes may feel dry after swimming.  This is due to the fresh 
(typically hypotonic) pool water trying to enter your cells (hypertonic) 
through these thin membranes.  The rest of the skin is a remarkably good 
barrier against the movement of water through it, regardless of what 
cosmetic sellers would imply; and not much fluid activity takes place 
through this organ.  The chlorine acts as an irritant adding to the dry 
feeling; in effect, it is trying to oxidize your cell surfaces.  A high 
level of chlorine can also create an osmotic gradient for the Cl- ions, and 
cause your cells to leach fresh water as the Cl- tries to enter them.  This 
high a level is unusual to find in all but the most hard-pressed public 
pools.  At these concentrations the oxidizer (chlorine, bromine, iodine) 
can be tasted as salt in the "fresh" pool water.  Keep in mind that in 
these pools, something else typically adds to the salinity.
 
Through the thin membranes (eyes, lips, intestines), salts (ions) are 
pulled out of the cells to try to add salts to the lower "salinity" pool 
water, while "fresh" pool water tries to enter your cells to dilute the 
salts present in them.  This causes a real problem when people aspirate 
water during a near or total drowning.  When you drown in fresh water, 
fresh water enters the cells of your lungs, and causes them to swell and 
rupture.  The resulting fluid release into the lungs can cause a dry land 
drowning several hours after the person has been removed from the water and 
was declared OK, and went home.  If one drowns in salt water, the higher 
salinity (usually) of the ocean causes the fresh(er) water to leach out of 
the lung cells into the salt ocean water, causing the cells to shrink.  If 
you survive, it is just a matter of rehydrating the cells (from the 
inside), and the damage is controlled.  So, given a choice (as if) it is 
much worse to drown in fresh water than salt water.

 Ions want to move from high gradients (hypertonic concentrations) to low 
gradients (hypotonic concentration).  This usually happens passively, 
requiring no expenditure of energy on part of the cells.  When this happens 
actively, the regulation of ion concentrations within the cells (at 
whatever concentration) is called osmoregulation.  If the water was the 
same salinity (relative ion concentration) as your body, this would be 
considered an isotonic solution, and there would be virtually no osmotic 
loss of fluids.  But you would still sweat, and lose moisture that way.  

(Bet you didn't know: Fresh water fish do not drink water, while salt water 
fish drink constantly.)

I suggest you contact the people in the DAN (Diver Accident Network) group 
(on the Internet), or a SCUBA Diving Instructors handbook (the usual 
student ones will barely mention this) if you need specific references you 
can quote or cite.

My references for this particular answer? Personal experience; many 
advanced SCUBA diving certifications, Hyperbaric Chamber Operations, 
Ichthyology, CPR and First Aid, Aquaculture, Chemistry, Anatomy and 
Physiology, and so on.  

I hope this was helpful, if you have any questions please contact me.

Good Luck!
Justin