Your body mainly consist of water (about 55% on average for females and 60% for males), being used in a variety of ways. About 50% of this water is used for shaping the cells. It is also used as a transport vehicle between the cells (42%) and in the bloodstream (8%). Water plays a vital role in regulating body temperature. First of al, it has a high specific heat capacity (4186 joules per kilogram Kelvin, higher than any other common substance), which means it does not easily heat up or cool down. In combination with the large quantity present in the body, this creates a large buffer capacity to absorb temperature changes. Finally, water is an important solvent for cellular metabolism and other processes in the body. These processes function less effective or simply collapse, when running short of water.
As you can seen, water affects every part of your body. When your body lacks water to function properly, you suffer dehydration. This means you risk being sick and eventually dying after several hours or days, if you do not take the appropriate actions. The precise duration for your demise depends on your physical condition and the environment. Count on a few hours in (very) hot weather and being (already) in a bad shape, even if you started out in top shape and well prepared. This conservative approach could save your life when Murphy strikes, especially if you forgot to think about him before setting out. It will also serve as a constant reminder of your vulnerable dependence on water. Dehydration can strike surprisingly quickly, turning you from hero to helpless in a few hours.
As a general rule, humans need about two litres of water a day to live healthily. Double that when doing exhausting, sweaty activities like hiking uphill with 20 kg on your back. Triple that when climbing at very high altitudes. Regular food could supply an important part of that amount (especially fruits), but drinking water plays a vital role her. Besides survival (also known as risk management), the most important issue on your vol bivouac is therefore likely to be water and the constant search for it. You can easily live for a few days without food (I tried it a few times), weeks if you are not doing exhausting things. Some people have survived weeks without eating, being trapped motionless under the debris of a collapsed structure for example, drinking water from a burst pipe or rain falling on the debris.
When you do not eat enough, your body will simply start to eat itself. Depending on your BMI, that could last a while. The higher your BMI (i.e. the more reserves you have), the longer you can live without eating. By burning your muscles and fat to stay alive, you won't be able to carry your heavy pack after a while or even walk in the final stages, but you can survive. Water on the other hand, is a different matter. Your body mainly consists of water, but can spare very little of it. The water present in your body is so essential for its functioning, that you can not trick with it. There is hardly any water reserve, and the reserve that is there could be used up quickly. For example, when sweating heavily. Your body is able to create energy by burning 'reserves' of fat and muscles, but it can not create water. It can only lose it. You will probably survive a few days without food, although this won't be a lot of fun, but you will end up dead after a few days without water.
This constant need for water is the major reason for my obsession with finding water. Always having my eyes open for a possible water source. Even when I have enough water, this reflex is hard to suppress. Any source is welcome, always. Not that I need it now, but I might need it later on. When running out of water for example, after failing to find any other (good) sources. Or maybe I will be somewhere near the (currently superfluous source) during another trip and remember its location when desperately needing water this time.
When your car's wheel bearings start squeaking like a chirping bird, you are too late with greasing them. Good maintenance usually means more mileage, with less (costly) repairs. When you have become thirsty, you are essentially too late with drinking. Your body has little capacity to store more water then required. Eating a lot makes you fat (i.e. builds reserves). Drinking a lot just makes you pee. The best way to maintain an adequate level of hydration is to drink little bits of water frequently, rather than large amounts upon becoming thirsty. And that is where a hydration pack comes in handy, also known as CamelBak, Dromedary Bag, Backcountry Bladder, Hydration Bag, or any other brilliant name that the marketing department came up with. Those manipulating marketeers might try to fool you into thinking that their brand is superior, justifying the higher price, because of some clever or cool advantage that is lacking in competitors' bags. However, all these bags are essentially nothing more than a water bag, with a hose and bite-valve to drink from. Very simple, but very effective items, more than worth their investment. The easy drinking from a hydration pack allows you to continuously compensate the water you lose by sweating. This means less water wasted and better overall health during your trip.
A hydration pack may look like am unnecessary contraption, invented for people not having the time or capability to drink from a bottle or cup, but it really helps to keep you hydrated. Drinking from a bottle requires that you (usually have to) stop what you are currently doing, reach or search for the bottle (possibly having to open your backpack or a pocket in the process), open it, tilt your head backwards, drink a bit, bring your head back to its regular position, repeat the last three actions a few times, close the bottle and put it back where it came from. You probably do not need an expensive management consultant specialised in Taylorism, in order to find out that there is room for efficiency improvements here. Drinking from a hydration pack requires you to open your mouth and bite in the valve that is hanging next to it. That is all.
Since most people prefer laziness over getting tired, even the simplest of actions require a commitment to do them correctly. The commitment to staying adequately hydrated is often forgotten as soon as the water bottle is no longer in view or within reach. People that choose to hike up a mountain with a heavy back pack out of their own free will, are far from lazy. Yet a large number of them somehow can not seem to find the trivial energy that is required to get the water bottle out of their backpack for a drink. They wait for a pause, when they have taken of their backpack. And even when the bottle is on the side of their backpack, within easy reach, they often seem to forget about it until they have become thirsty. I admit to having been one of these people a long time ago, but I would hate to fall back on the bottle should my hydration pack expire unexpectedly during a trip. Ridiculous as it may look, having a hose next to your mouth greatly reduces drinking problems.
My hydration pack is a Dromedary Bag from MSR. It is a simple robust bag and lacks the straps and fabric that allow you to carry it like a little backpack. I have no need for those, for it gets stored in the big backpack. It also lacks the tendency to turn water into a plastic tasting substance after a few hours, which some of the hydration packs tend(ed?) to do. The cordura and nylon used for the Dromedary Bag are not lightweight stuff, but light enough and very durable. My bag has (accidentally) endured quite a bit of rough handling over the years, and is still going strong. The cap lasts not as long as the bag, and has been replaced twice in eight years or so. The bite valve normally lasts a season before the leaking becomes too annoying, but this could be less than a month if I am less lucky. Not very surprising, considering that the bite valve is the only item that actually sees frequent mechanical stress. My bite valve is from CamelBak, because the MSR ones sucked. They felt clumsy when drinking and started dripping a lot faster than their sturdy looks suggested. The CamelBak bite valve looks fragile, but is an example of simplicity and effectiveness. By pure coincidence of course, these happen to be my main criteria for good functional design. The CamelBak bite valve sits on top of a shut off valve, which comes in handy for preventing accidental water loss. For example when turning over your backpack and not noticing that you buried the bite valve underneath.
Occasionally, I have met people that were not convinced of the vital importance of frequently drinking sufficient qualities of water. These people refuse to drink while flying for example, mostly in order to avoid pissing. Or they refuse to see the point of drinking, since the water they drink will be lost by sweating anyway. It could be that that these people have bodies that behave different from most average human beings, but I suspect that asking (for example) their kidneys for an opinion on this issue, would confirm the overall hydration opinion. But let's not get carried away by prejudice here. Maybe those none-drinkers could be right? Not having to carry all that water would save us a lot of weight and hassle during our vol bivouac. Let's have a look at our bodies to see if we can learn something from these none-drinkers.
Your body gathers energy from the food you are eating. This energy is used for heating and powering your body. If their is more food input than needed, the energy is stored as a reserve for less abundant times. All cells in your body require energy to function, but let's focus on the main issue here: your muscles. Muscles burn calories when exercised, which causes your body temperature to rise. Your body needs to get rid of this surplus temperature, otherwise it will become overheated and stop working. The body has several mechanisms for losing warmth, such as by the air you exhale, by your blood being cooled when passing under your skin, and by sweating of course. Since all mechanisms depend on exchange with the environment, ambient temperature has a large influence on their effectiveness. Radiation is the principle mechanism at room temperature, not sweating. But even though you do not seem to be sweating at this temperature, you are losing water through your skin. As the ambient temperature rises, your body relies more and more on water evaporation to stay cool and you noticeably start sweating. If the ambient temperature approaches your body temperature, cooling mechanisms such as radiation, conduction, and convection simply stop. At higher temperatures, these mechanisms even reverse and start heating up your body instead of cooling it. The most important thing to remember here is that in high ambient temperatures and/or demanding physical effort, sweating becomes the principal cooling mechanism. This cooling mechanism will degrade and eventually fail, if your body lacks sufficient water for sweating. This will result in a rising body temperature, causing a thermal breakdown if you fail to act properly and instantly.
Humidity has a major influence on the effectiveness of sweating. Saturated air prevents sweat from evaporating, causing it to collect on your skin and run off rather than cool. This is probably the effect that none-drinkers mean when they say that drinking does not help cooling them and only makes them sweat. Well, even though the sweating might not really be cooling you, it is causing lots of water to leave your body. You are better off replacing those lost quantities by drinking, rather than letting your body get it from somewhere more vital.
Heat-related illnesses are greatly accelerated by dehydration. In a dehydrated body, blood has thickened, forcing organs to work harder. The heart for example, has more work pumping around the ticker blood (more friction), through blood vessels that have narrowed (yet more friction) in order to adapt to the decreased blood volume. Having less blood volume also means a decreased cooling effectiveness of the blood near the skin. System engineers among the readers probably recognize this as a system with positive feedback. It is an unstable system, getting from bad to worse in a very short time. As soon as dehydration sets in, your body starts acting weird in no-time.
You could be walking along tired but happily at the end of a long day, knowing that there is a rewarding refuge in about thirty minutes of level walking or so, when you suddenly start feeling a little faint. You are not surprised, since the last hike up was steep and tough, but you guess things will get better soon now that you are finally walking level. Instead, the dizzy feeling is followed by nausea a few minutes later as you keep on walking. You drink a bit more, but this is not helping much. On the contrary, it makes you feel a bit sick. Your muscles were already tired, but now start aching as well. You keep on walking, since you are almost there. The path goes a little uphill. It is just a little and you would not really have noticed it when in a healthy shape, but now it feels like climbing the summit of Mont Blanc. You have become so tired all of a sudden that you can hardly walk. You sit down to rest a bit, when suddenly you feel sick and throw up. After a few minutes you feel better and seem to have regained a little strength in order to walk again. A few minutes later and 200 m further down the path, the vomiting and resting pattern repeats itself. It keeps on repeating and you feel close to collapsing, when you finally have the refuge in sight. Just a few hundreds of metres! But you have hardly any energy left to move, let alone walk. The vomiting distance has shortened to about 50 m, with five minutes recovery time on average. It is getting dark. Time flies when you are not having fun, especially when you need it most. You hesitate between continuing your struggle to the refuge or sleeping here on the path until you feel better. You decide against sleeping, since you are not really sure of waking up again. You suddenly realize that you might need help, so you move on. You finally reach the last hurdle: that narrow bridge that leads up to the refuge. It looked long from a distance, but now it seems endless. You realise that you risk drowning if you fall or faint, since the bridge is only half a metre wide and on one side there is no barrier to stop you from falling into the water. There is only a single hand-rail to steady you as you shuffle to the other side, careful not to step next to the bridge and occasionally looking at the waterfall below your feet. Luckily, there is no oncoming traffic. There is no room to pass. The fast flowing water, visible between the steps under your feet, starts its siren song.
With a final effort you reach the refuge, located a little above the bridge. You are still cursing yourself for being such an idiot to have ended up in this situation, as you continuously did during the past hours, but the cursing now has a happy edge to it. But wait a minute, were is the entrance? Is the refuge closed!? As you stumble around the building looking for the entrance, the refuge's warden comes out of the kitchen door and guides you to the now evident entrance you completely overlooked. A few hours later, everything is fine. After a good night's sleep, you feel like taking on the world again. Remembering yesterdays ordeal however, you decide to take it a bit easier today as you continue your trip.
Unfortunately, the above scenario is from personal experience. Maybe there are other unpleasant symptoms as well, but I would hate to find them out. I learned about dehydration the hard way, needing a few lessons to really understand the warning signals and dangers. The common cause for discovering the above symptoms was the usual combination of overestimating my endurance and luck, and underestimating the consequences thereof. Especially the speed at which these symptoms arrived surprised me, even when I knew they were coming. More severe cases of dehydration could even result in permanent damage to about any body part that relies on fluids for its functioning. And yes, that includes the brain. Which could explain the reason for the existence of none-drinkers, as well as my lack of understanding them. Some people are more resistant to dehydration and heat-related illnesses than others. However, being a non-drinker is probably more about fooling yourself than about being more resistant.
To summarise all the above: drink enough water to remain hydrated and reduce or stop sweaty activities when you are loosing more water than you are drinking. Try to guess your level of dehydration before the symptoms set in, by monitoring your water intake (e.g. remember the quantities you drank in the last hour, frequently check the water level of your hydration pack). Especially pay attention in hot and humid conditions since these (greatly) reduce the effectiveness of sweating. Remember that hot and humid often go together, since hot air is able to hold more moisture than cold air. Lessen your desires, but especially your pace, when confronted with these kind of conditions. By taking things easy, you will arrive at your destination eventually. By speeding you might get there sooner, or not at all. If you (in case of an emergency have to) continue with an activity that is likely to cause dehydration or heat-related illness, then remember that your body temperature starts to rise quickly when these set in, so plan your objectives accordingly.
When the symptoms do appear nonetheless, then act promptly before they get worse. The most important action is to stop any further loss of fluids (sweating, vomiting, diarrhoea), which usually means stopping your activity. Rest and drink water instead. Try to find some shade or other cool place to rest if ambient temperatures are high. Do not eat until you feel a bit better, since you risk vomiting that could cause further loss of fluids. If you already feel sick, then eating is likely to trigger vomiting or increase the risk thereof. In my case, the quickest road to recovery usually consists of resting, while drinking tiny bits of water frequently. As soon as I stop walking, the (urge for) vomiting stops. This means my body takes up the water I drink, rather than spitting it out. When the nausea stops, I will start eating tiny bits of easily digestible food such as dried raisins or a biscuit. This usually calms my stomach and strengthens my body a bit. The dried raisins I eat consist of 65% fructose (i.e. a monosaccharide, thus easily digested by the body), and lots of vitamins and minerals as well, giving my body that much needed boost. Remember that you lose more than water alone when sweating, such as salt. This loss need to be replenished too. When this tiny amount of food stays in, small quantities of more substantial food follow. Usually slow-acting carbohydrates to help my body recuperate in the coming hours. If not, then I just keep on drinking tiny bits and sleep for a few hours. Sleep is a great healer for a tired body. A tired body is more vulnerable, especially in dehydrated state.
All of what I have written here helps me in staying hydrated, as well in recovering should dehydration occur nonetheless. There could be more that works even better for you. Spent a little time learning about dehydration and heat-related sickness before setting out. It will be time well spent. And remember that caffeine and alcohol drain more water from your body than you gain by these liquids, increasing the risk of dehydration. Some drugs and medications could do the same. Think about that when taking a bottle of wine along on your vol bivouac, rather than pondering over its weight.