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For other uses, see Dehydration (disambiguation).
Classification and external resources
10 9 MedlinePlus 000982

In physiology and medicine, dehydration (hypohydration) is the excessive loss of body water,[1] with an accompanying disruption of metabolic processes. It is literally the removal of water (Ancient Greek: ὕδωρ hýdōr) from an object; however, in physiological terms, it entails a deficiency of fluid within an organism. Dehydration of skin and mucous membranes can be called medical dryness.

Some authors have reported three types of dehydration based on serum sodium levels: hypotonic or hyponatremic (referring to this as primarily a loss of electrolytes, sodium in particular), hypertonic or hypernatremic (referring to this as primarily a loss of water), and isotonic or isonatremic (referring to this as equal loss of water and electrolytes).[2] In reality however, it emerges that by "hyponatremic" and "eunatremic" dehydration these authors are actually referring to "hypovolemia" (see definition below). In humans, it is thought that the most commonly seen type of dehydration by far is isotonic (isonatraemic) dehydration, but this effectively refers to hypovolemia. "Dehydration", is thus a term that has loosely been used to mean loss of water, regardless of whether it is as water and solutes (mainly sodium) or free water. Those who refer to hypotonic dehydration therefore refer to solute loss and thus loss of intravascular volume but in the presence of exaggerated intravascular volume depletion for a given amount of total body water gain. It is true that neurological complications can occur in hypotonic and hypertonic states. The former can lead to seizures, while the latter can lead to osmotic cerebral edema upon rapid rehydration.[3] It is thus important to distinguish "dehydration" from "hypovolemia" and maybe limit the term "dehydration" to states of "hypernatremia" and call all other usage "hypovolemia" as that would greatly facilitate management.


Hypovolemia is specifically a decrease in volume of blood plasma.[4][5] and dehydration is specifically a decrease in total body water, that for the most part spares blood plasma volume. Humans can lose water from the body — of any aetiology — but that loss can only occur in two distinct patterns with separate mechanisms:

  1. Loss of blood volume
  2. Loss of tissue water

Not only do both have different manifestations, the mechanism of the water loss that leads to a) or b) is distinct. a) can only occur if sodium is lost and the water loss is secondary to this sodium loss and mainly blood plasma water. b) can only occur if pure water loss is independent of sodium loss and mainly derived from intracellular water. Both can co-exist i.e. sodium and water loss that is not balanced i.e. more water loss than expected for sodium loss. Dehydration then refers to pure (tissue) water loss and hypovolemia to sodium loss and thus loss of blood volume. This is the correct and pure definition[6] but unfortunately gets wildly mixed up. Very interestingly the opposite can also occur i.e. overhydration as well as hypervolemia. Finally they do not need to move in the same direction i.e. a person can be hypervolemic and dehydrated at the same time and any combination of these six states can coexist — if we count euvolemia and normal hydration.

Signs and symptoms

Symptoms may include headaches[7] similar to what is experienced during a hangover, decreased blood pressure (hypotension), and dizziness or fainting when standing up due to orthostatic hypotension. Untreated dehydration generally results in delirium, unconsciousness, swelling of the tongue and, in extreme cases, death.

Dehydration symptoms generally become noticeable after 2% of one's normal water volume has been lost. Initially, one experiences thirst and discomfort, possibly along with loss of appetite and dry skin. This can be followed by constipation. Athletes may suffer a loss of performance of up to 30%[8] and experience flushing, low endurance, rapid heart rates, elevated body temperatures, and rapid onset of fatigue.

Mild dehydration, which includes water losses between 1% and 2%, observed in the experiment are comparable to mild dehydration experienced by people in their everyday lives.

In moderate to severe dehydration, there may be no urine output at all. Other symptoms in these states include lethargy or extreme sleepiness, seizures, sunken fontanel (soft spot) in infants, fainting, and sunken eyes.

The symptoms become increasingly severe with greater water loss. One's heart and respiration rates begin to increase to compensate for decreased plasma volume and blood pressure, while body temperature may rise because of decreased sweating. At around 5% to 6% water loss, one may become groggy or sleepy, experience headaches or nausea, and may feel tingling in one's limbs (paresthesia). With 10% to 15% fluid loss, muscles may become spastic, skin may shrivel and wrinkle (decreased skin turgor), vision may dim, urination will be greatly reduced and may become painful, and delirium may begin. Losses greater than 15% are usually fatal.

In people over age 50, the body’s thirst sensation diminishes and continues diminishing with age. Many senior citizens suffer symptoms of dehydration. Dehydration along with hyperthermia results in the elderly dying suddenly during extreme hot weather.

Diseases of the gastrointestinal tract can lead to dehydration in various ways. Often, dehydration becomes the major problem in an otherwise self-limited illness. Fluid loss may even be severe enough to become life-threatening.

In studies of terminally ill patients who have chosen to die, it has been shown that deaths by terminal dehydration are generally peaceful, and are not associated with suffering when supplemented with adequate pain medication.[10][11]

Differential diagnosis

In humans, dehydration can be caused by a wide range of diseases and states that impair water homeostasis in the body. These include:

Other causes of obligate water loss


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.[13][14][15][16][17]

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 (about 350ml), through the skin by perspiration (100ml) and by diffusion through the skin (350ml), or through the kidneys as urine (1000–2000ml, about 900ml of which is obligatory water excretion that gets rid of solutes). Some water (about 150–200ml, in the absence of diarrhea) is also lost through the bowels.[18] In warm or humid weather or during heavy exertion, however, the water loss can increase by a factor of 10 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[15] 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 mechanisms such as vomiting or diarrhea, an imbalance can develop very quickly into a medical emergency.

During sports events such as marathons, athletes take frequent water stops and water breaks to avoid dehydration. The claim that pure water without isotonic additives can prevent dehydration is questioned, however, and the European Food Safety Authority states that reduced body water content is only a symptom of dehydration and not what drinking water controls.[19] Drinking water without replenishing electrolytes may instead lead to developing hypotonic dehydration.


Main article: Management of dehydration

The treatment for minor dehydration, often considered the most effective, is drinking water and stopping fluid loss. Plain water restores only the volume of the blood plasma, inhibiting the thirst mechanism before solute levels can be replenished.[20] Solid foods can contribute to fluid loss from vomiting and diarrhea.[21]

In more severe cases, correction of a dehydrated state is accomplished by the replenishment of necessary water and electrolytes (through oral rehydration therapy or fluid replacement by intravenous therapy). As oral rehydration is less painful, less invasive, less expensive, and easier to provide, it is the treatment of choice for mild dehydration. Solutions used for intravenous rehydration must be isotonic or hypotonic. Pure water injected into the veins will cause the breakdown (lysis) of red blood cells (erythrocytes).

When fresh water is unavailable (e.g., at sea or in a desert), seawater and alcohol will worsen the condition. Urine contains a similar solute concentration to seawater, and numerous guides advise against its consumption in survival situations. [22][23][24][25]

For severe cases of dehydration where fainting, unconsciousness, or other severely inhibiting symptom is present (the patient is incapable of standing or thinking clearly), emergency attention is required. Fluids containing a proper balance of replacement electrolytes are given orally or intravenously with continuing assessment of electrolyte status; complete resolution is the norm in all but the most extreme cases.

Some research indicates that artificial hydration to alleviate symptoms of dry mouth and thirst in the dying patient may be futile.[26]

See also


  • Patient Refusal of Nutrition and Hydration: Walking the Ever-Finer Line." American Journal Hospice & Palliative Care, pp. 8–13. (March/April 1995)


External links

  • Definition of dehydration by the U.S. National Institutes of Health's MedlinePlus medical encyclopedia
  • Rehydration Project at
  • About dehydration
  • Dehydration Causes, Symptoms and Treatment

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