Excerpted from Medicine For the Backcountry by Buck Tilton M.S. & Frank Hubbell D.O.

We assume that our thirst mechanism will protect us from dehydration. That if we become dehydrated, it will tell us by triggering intense thirst until we are properly hydrated. But this isn't always the case. It is possible to lose fluid so quickly that the normal thirst mechanism is overwhelmed or overridden.

The sources of fluid loss are respiration, perspiration, urination and defecation. The rate of loss from each of these will vary according to activity levels, air temperature, humidity and altitude.

RESPIRATION: With normal daily activities we lose approximately one to two liters of water via evaporation from the lungs. As we breathe, the air that we inhale is"conditioned"-- it is warmed to 98.6F and humidified to 100% humidity. This warm, moist air is then exhaled and the cycle starts over again. The combination of extreme cold temperatures and altitude can dramatically increase the rate of water loss through the lungs -- up to one cup per hour, or six liters in 24 hours.

PERSPIRATION: With normal daily activities we do not sweat very hard so loss is minimal -- about one to two liters per day. But with exertion in hot, dry weather, loss through sweating can become extreme. During heavy exertion, we can lose one to three liters of water per hour, which can easily amount to 8-10 liters of fluid over an afternoon of exercise.

URINATION: One to two liters of fluid are lost daily via urination -- a natural clearing of waste products from the blood. The amount of urine produced will increase with overhydration and decrease with dehydration.

DEFECATION: The average daily loss of fluid through defecation is only abour 0.1 liter. However, this can increase dramatically with diarrhea to as much as 25 liters over a 24-hour period.

 

	Combatting Heat -- Rules of Thumb:   1. Stay well hydrated and eat salty snacks. 2. Rest often, out of the sun. 3. Wear clothing that allows evaporation and a brimmed hat or cap. 4. Give heat exhaustion patients lots of water with a teaspoon of salt per liter. Let them rest. 5. Cool heat stroke patients as rapidly as possible. Douse with water, fan, and massage extremities. Evacuate as soon as possible.

Through the activities of daily living, the average day's loss of fluid is four liters, which is generally replaced by the fluid we drink and the food we eat. But the amount of fluid required can be significantly increased by exercise, sweating, diarrhea, temperature or altitude. The most common cause of increased fluid loss is exercise and sweating.

John, Jan and David thought they were prepared with their one liter of water each. They did not appreciate the amount of fluid that they would lose from sweating, or the effects that dehydration would have on them. By mid afternoon, when David began to complain, they were already dehydrated and suffering the consequences.

The effects of even mild dehydration are decreased coordination, fatigue, and impairment of judgment. These factors increased the difficulty of of their hike out and could have lead to a physical mishap or getting lost.