Dehydration (hypohydration) is the removal of water (hydro in ancient Greek) from an object. Medically, it is a condition in which the body contains an insufficient volume of water for normal functioning.
Medical causes of dehydration in humans
In humans, dehydration can be caused by a wide range of diseases and states that im
pair water homeostasis in the body. These include:
- External or stress-related causes
- Prolonged physical activity without consuming adequate water, especiall
y in a hot and/or humid environment
- Prolonged exposure to dry air, e.g. in high-flying airplanes (5-15% relative humidity)
- Survival situations, especially desert conditions
- Blood loss or hypotension due to physical trauma
- Shock (hypovolemic)
- Use of Methamphetamine
- Infectious diseases
- Yellow fever
- Electrolyte disturbance
- Hypernatremia (also caused by dehydration)
- Hyponatremia, especially from restricted salt diets
- Recent rapid weight loss may reflect progressive depletion of fluid volume (the loss of 1 L of fluid results in a weight loss of 1 kg or 2.2 lb).
- Patient refusal of nutrition and hydration
- Other causes of obligate water loss
- Severe hyperglycemia, especially in Diabetes mellitus
Symptoms and prognosis
Symptoms may include headaches similar to what is experienced during a hangover, a sudden episode of visual snow, 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%, and experience flushing, low endurance, rapid heart rates, elevated body temperatures, and rapid onset of fatigue.
Symptoms of mild dehydration include thirst, decreased urine volume, abnormally dark urine, unexplained tiredness, lack of tears when crying, headache, dry mouth, and dizziness when standing due to orthostatic hypotension.
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.
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, vision may dim, urination will be greatly reduced
and may become painful, and delirium may begin.
Losses greater than 15%
are usually fatal.
Dehydration is best avoided by drinking plenty of 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 water frequently). Drinking
water beyond the needs of the body entails little risk, 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 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. In fact, the main mechanisms through which diseases such as infantile diarrhea and cholera kill their victims are dehydration and loss of electrolytes.
During sports events, water stops and water breaks are provided to avoid dehydration of athletes.
The best treatment for minor dehydration is drinking water and
stopping fluid loss. Water is preferable to sport drinks and other
commercially-sold rehydration fluids, as the balance of lectrolytes
they provide may not match the replacement requirements of the
individual.To stop fluid loss from vomiting and diarrhea, avoid solid foods and drink only clear liquids.
In more severe cases,
correction of a dehydrated state is accomplished by the replenishment
of necessary water and electrolytes (rehydration, through oral
rehydration therapy or intravenous therapy). Even in the case of
serious lack of fresh water (e.g., at sea or in a desert), drinking
seawater or urine does not help, nor does the consumption of alcohol.
It is often thought that the sudden influx of salt into the body from
seawater will cause the cells to dehydrate and the kidneys
to overload and shut down but it has been calculated that an average
adult can drink up to 0.2 liters of seawater per day before the kidneys
start to fail. There is a Xavier Manguet theory that a adult organism
can resist for a week by drinking low amounts of salt water before
kidney die, and in that case a survival is active and conscious all the
week long period.
unnecessary sweating should be avoided, as it wastes water. If there is
only dry food, it is better not to eat, as water is necessary for
digestion. For severe cases of dehydration where fainting,
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.