Dehydration (hypohydration) is the removal of water (hydro in ancient Greek) from an object. In physiological terms, it entails a relative deficiency of water molecules in relation to other dissolved solutes. Dehydration, thus, is slightly different from hypovolemia, which defines water deficiency only in terms of overall volume rather than in terms of solute concentrations.

Medical causes of dehydration in humans

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 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
• Diarrhea
• Hyperthermia
• Shock (hypovolemic)
• Vomiting
• Burns
• Lacrimation
• Use of Methamphetamine, Amphetamine and other stimulants.
• Drinking of Alcohol.

• Infectious diseases
• Cholera
• Gastroenteritis
• Shigellosis
• Yellow fever

• Malnutrition
• Electrolyte disturbance
• Hypernatremia (also caused by dehydration)
• Hyponatremia, especially from restricted salt diets
• Fasting
• 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
• Glycosuria

Symptoms and prognosis

Symptoms may include headaches similar to what is experienced during a hangover, muscle cramps, 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 tongue1 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, irritability, lack of tears when crying, headache, dry mouth, dizziness when standing due to orthostatic hypotension, and in some cases can cause insomnia.