Dehydration is the removal of water (hydor in ancient Greek) from an object. There are many methods of dehydration, with the most common being the application of dry heated air. This causes evaporation of the surface water, which is replaced by water internally.

Drying is often used as a method of preserving food and to obtain absolute alcohol.

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

Examples are:

• infectious diseases
  • yellow fever
  • cholera
  • Shigellosis
  • common gastrointestinal viral and bacterial infections resulting in vomiting and diarrhea, also known as gastroenteritis.

• external or stress related causes
  • Hyperthermia
  • shock
  • blood loss or hypotension due to trauma
  • prolonged physical activity without consuming adequate water, especially in a hot environment

• dietary imbalances due to
  • nausea
  • restricted salt diets
  • excessive consumption of alcohol

• other causes of obligate water loss or electrolyte abnormalities
  • severe hyperglycemia in diabetes. Excretion of glucose with urine (glucosuria) is accompanied with water loss. Water is the required solvent.

Symptoms may include headaches similar to what is experienced as a hangover, and lower blood pressure and a dizzy feeling when standing up which is called orthostatic hypotension. Extreme cases of dehydration can result in unconsciousness and death.

Correction of a dehydrated state is performed by rehydration, and the addition of necessary electrolytes.

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, neither does the consumption of diuretics such as caffeine and alcohol. Unnecessary sweating should be avoided, it wastes water. If there is only dry food, it is better not to eat, as it needs water to aid digestion.

Avoiding dehydration

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 vapour, through the skin as sweat, or through the kidneys as urine. Some (a less significant amount, in the absence of diarrhea) is also lost through the bowels.

When taking vigorous exercise or visiting a hot country, it is easy to lose twice this amount. Heavy exercise in high temperatures could cause the loss of over 2.5 litres of fluid per hour, which exceeds the body's absorptive capacity.

See also

• Desert survival

External links

• Tips from the SAS Survival Guide by John Wiseman (http://www.acs.ucalgary.ca/~smzess/survival/water.htm)
• Report of a navy officer including some tips (http://www.safetycenter.navy.mil/media/approach/issues/mar02/cool.htm)
• Are you drinking too much water? (http://www.ivillage.co.uk/health/hlive/eat/articles/0,,181168_588681,00.html)