The cells in your body has a phospholipid bilayer which allows water to move freely from outside to inside or vise versa. This membrane does not freely allow sodium or potassium to cross through (that requires energy in the sodium potassium pump). Due to osmosis water will move from higher concentration to lower concentration to reach equilibrium between the extracellular fluid and intracellular fluid. So if you eat a bunch of salt without any water the increased sodium outside your cells will cause water to more from inside to outside your cells to try and equalize the osmolarity. Conversely, if you load way up on water your cells can swell because the excess water outside the cells will enter into the cell to reach a stable balance.
It can get a bit more complicated with different types of cells such as skeletal muscle cells and activity, in this situation sodium, potassium, and calcium all play essential roles in moving in and outside of cells quickly to create the action potentials (actual contraction of the muscles). Then we can digress into different energy systems and the various demands etc, but the point is the body is very good at maintaining chemical equilibrium at the cellular level.
Also your Renal system (kidneys) can significantly alter your fluid levels in the blood plasma through different hormones to help maintain that proper equilibrium. This is mostly related to maintaining proper blood pressure/osmolarity/pH, but again will influence the amount of fluids the body retains. As another poster pointed out hormones like ADH (Vasopressin), aldosterone, renin-angiotensin, and ANP (Atrial naturiuretic peptide) all significantly alter the bodies fluid levels to maintain very tight tolerances.
My takeaway is our bodies are very good at maintaining equilibrium assuming i do not do something stupid, like not drink at all during workouts or drink 40oz an hour and nothing else. The “nice” think about true dehydration is it will set on quickly and you will know it, when you are truly dehydrated the effects are unmistakable. I had the state road race championships this year and it was warmer than normal (90 by 11am) and i missed the feed at mile 40 of 80 miles, i just thought i would be fine not having any fluids the last 35 miles. I started to notice around mile 70 i stopped sweating, my arms looked really small, skin just looked different, and started to cramp everywhere, by mile 75 i could not move any muscle without it cramping. Barely made in back and it took around 4-5 hours to really recover and start to feel normal.
Hyponatremia (very low sodium levels caused most often by drinking lots of water) is a very serious condition that is not as easy to detect as dehydration. If you have time try and read some medical articles on this, most are fairly “easy” to understand and very enlightening. Lot’s of medical cases related to endurance athletes who drink so much water and take in no additional sodium with pretty serious medical consequences. The most common end game to this is serious brain swelling leading to a coma or death.
The body is great at maintaining balance… in the long-ish term.
But, those systems cannot react quickly enough to short-term stimulus. This is why the pre-hydration products work (from Skratch, Osmo, etc) to increase plasma volume.
Agree. Pre-Hydration/ hyperhydration have been around for quite a while and work if done properly.
http://www.ncbi.nlm.nih.gov/pubmed/18838842
“Pre-exercise hyperhydration significantly increased time to exhaustion and peak power output, compared with pre-exercise euhydration. We conclude that pre-exercise hyperhydration improves endurance capacity and peak power output and decreases heart rate and thirst sensation”
