(From the British magazine "High Mountain", Jan 2004, by I think Andy Kirkpatrick)
Moisture is lost through 3 main methods – sweating, breathing, and excretion.
Much of this moisture is drawn from the space between the cells, which is restored by blood plasma. If you are not replacing the water, blood volume will decrease as this plasma is used up, lessening your performance.
Less blood returning to your heart means the heart needs to pump faster to maintain the same cardiac pressure.
This decreases oxygen flow to your muscles and reduces the removal of carbon dioxide and lactic acid, causing cramps and increased muscle fatigue.
In cold conditions, this reduction in circulation can lead to cold extremities, which is a big factor in frostbite.
The cold itself also compounds dehydration due to moisture loss caused by heating up cold air while breathing (up to 1.5 liters per hour).
So, drink up!
Try to drink at least half a liter 2 hours before exercising, to promote hydration and allow time for excretion of excess water. (For a Hood climb, this means before you leave home.)
In colder weather, it’s best if the liquid is lukewarm. At the trailhead, you can pour hot water from a thermos into your bottles to start off with warm water.