Dehydration (''hypohydration'') is defined as excessive loss of body fluid. It is literally the removal of water from an object, however in physiological terms, it entails a deficiency of fluid within an organism.
There are three main types of dehydration: hypotonic (primarily a loss of electrolytes, sodium in particular), hypertonic (primarily a loss of water), and isotonic (equal loss of water and electrolytes). In humans, the most commonly seen type of dehydration by far is isotonic (isonatraemic) dehydration which effectively equates with hypovolemia, but distinction of isotonic from hypotonic or hypertonic dehydration may be important when treating people who become dehydrated.
Hypovolemia is specifically a decrease in volume of blood plasma. Furthermore, hypovolemia defines water deficiency only in terms of volume rather than specifically water. Nevertheless, the conditions usually appear simultaneously.
Physiologically, it is important to understand that dehydration, despite the name, does not simply mean loss of water, as water and solutes (mainly sodium) are usually lost in roughly equal quantities to how they exist in blood plasma.
Dehydration is best avoided by drinking sufficient water. The greater the amount of water lost through perspiration, the more water must be consumed to replace it and avoid dehydration. Since the body cannot tolerate large deficits or excesses in total body water, consumption of water must be roughly concurrent with the loss (in other words, if one is perspiring, one should also be drinking some water frequently).
For routine activities in which a person is not perspiring to any large degree, drinking when one is thirsty is sufficient to maintain hydration. However, during exercise, relying on thirst alone may be insufficient to prevent dehydration from occurring. This is particularly true in hot environments, or for those older than 65. For an exercise session, an accurate determination of how much fluid is necessary to consume during the workout can be made by performing appropriate weight measurements before and after a typical exercise session, to determine how much fluid is lost during the workout.
Drinking water beyond the needs of the body entails little risk when done in moderation, since the kidneys will efficiently remove any excess water through the urine with a large margin of safety.
A person's body, during an average day in a temperate climate such as the United Kingdom, loses approximately 2.5 litres of water. This can be through the lungs as water vapor, through the skin as sweat, or through the kidneys as urine. Some water (a less significant amount, in the absence of diarrhea) is also lost through the bowels. In warm or humid weather or during heavy exertion, however, the water loss can increase by an order of magnitude or more through perspiration; all of which must be promptly replaced. In extreme cases, the losses may be great enough to exceed the body's ability to absorb water from the gastrointestinal tract; in these cases, it is not possible to drink enough water to stay hydrated, and the only way to avoid dehydration is to either pre-hydrate, or find ways to reduce perspiration (through rest, a move to a cooler environment, etc.)
A useful rule of thumb for avoiding dehydration in hot or humid environments or during strenuous activity involves monitoring the frequency and character of urination. If one develops a full bladder at least every 3-5 hours and the urine is only lightly colored or colorless, chances are that dehydration is not occurring; if urine is deeply colored, or urination occurs only after many hours or not at all, water intake may not be adequate to maintain proper hydration.
When large amounts of water are being lost through perspiration and concurrently replaced by drinking, maintaining proper electrolyte balance becomes an issue. Drinking fluids that are hypertonic or hypotonic with respect to perspiration may have grave consequences (hyponatremia or hypernatremia, principally) as the total volume of water turnover increases.
If water is being lost through abnormal mechanisms such as vomiting or diarrhea, an imbalance can develop very quickly into a medical emergency.
During sports events such as marathons, water stops and water breaks are provided to avoid dehydration of athletes.
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