Dehydration (hypohydration) is the removal of water (hydor in ancient Greek) from an object.
Medically, dehydration is a serious and potentially life-threatening condition in which the body contains an insufficient volume of water for normal functioning. The term "volume depletion" is similar to dehydration, but it refers to the loss of salts as well as water. Also see Hypovolemia.
In chemistry, dehydration refers to a type of elimination reaction that involves the loss of water ( H2O) from a molecule.
Medical causes of dehydration
In humans, dehydration can be caused by a wide range of diseases and states that impair water homeostasis in the body. These include:
External or stress-related causes
· Prolonged physical activity without consuming adequate water, especially in a hot environment
· Prolonged exposure to dry air, e.g., in high-flying airplanes (5-15% r.h.)
· Survival situations, especially desert survival conditions
· Blood loss or hypotension due to physical trauma
· Infectious diseases
· Yellow fever
· Electrolyte imbalance
· Hypernatremia (also caused by dehydration)
· Hyponatremia, especially from restricted salt diets
· Excessive consumption of alcohol
· 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 as 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, and 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. Athletes may suffer a loss of performance of up to 50%, 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, urine that is darker than usual, 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 will 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 of greater than 15% are usually fatal
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 average adult can drink up to 0.2 liters of seawater per day before the kidneys start to fail. 
When dehydrated, 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. The best treatment for minor dehydration is water; sport drinks and other fluids commercially sold for rehydration should be used with care, as the balance of electrolytes they provide may or may not match the replacement requirements of the individual. For severe cases of dehydration where fainting, unconsciousness, or any 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 intervenously with continuing assessment of electrolyte status; complete resolution is the norm in all but the most extreme cases.
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 slightly beyond the needs of the body entails no risk, since the kidneys will efficiently remove any excess water through the urine with a large margin of safety.
A person's body loses, during an average day in a temperate climate such as the United Kingdom, 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 that carry away electrolytes in large quantities, electrolyte imbalance can develop very quickly into a medical emergency. The main mechanisms through which diseases such as infantile diarrhea and cholera kill their victims are dehydration and loss of electrolytes. In some cases, simple oral administration of clean water with a balanced supplement of replacement electrolytes can be life-saving.