Water is one of the most important elements in wilderness survival. Even a slight change in the balance of water in our bodies can bring on adverse physical and mental effects. Dehydration can cause depression, poor judgment, slowed muscle response, nausea, and a host of other problems that can impair survival ability.
Water helps regulate temperature, is the basis of digestion and metabolism, lubricates joints, helps eliminate wastes, and is needed for breathing (lungs need moisture).
Dehydration is deadly in hot and cold weather. When the blood in your circulatory system loses water, it gets thicker. Thick blood circulates more slowly and is harder for the heart to pump, and, in regard to temperature regulation, hinders the body's ability to lose excess heat or circulate needed heat. When the volume of blood and extra cellular fluids decreases, water is literally sucked from the cells causing them to shrink and damaging cell membranes and the proteins inside.
Body functions are severely limited if you lose 10 percent of your weight due to dehydration yet physical, mental, and emotional impairment manifest with the slightest loss of water, especially in the heat. A loss of just 2 percent of body weight in water compromises your overall judgment by 25%. Being thirsty means you are already low.