Refers to excessive loss of body water. 



In physiology, dehydration is a deficit of total body water, with associated disruption of metabolic processes. 



Dehydration occurs when free water loss exceeds free water intake.



Dehydration is usually due to exercise, disease, or high environmental temperature. 



Most people can tolerate a 3-4% decrease in total body water without difficulty or adverse health effects. 



A 5-8% decrease in body water can cause fatigue and dizziness. 



Loss of over ten percent of total body water can cause physical and mental deterioration,.



Loss of over ten percent of total body water is associated with severe thirst.



Death from dehydration occurs at a loss of between fifteen and twenty-five percent of the body water.



Mild dehydration is associated with thirst and general discomfort.



Mild dehydration is usually resolved with oral rehydration.



It can cause hypernatremia and hypovolemia



Dehydration manifests as thirst and neurological changes such as headaches, general discomfort, loss of appetite, decreased urine volume, confusion, unexplained tiredness, purple fingernails and seizures. 


May be associated with thirst, dry mouth, decreased urine output, electrolyte imbalance, asthenia, cramps, headaches, nausea, vomiting, postural hypotension, delirium, cognitive failure, renal failure and somnolence.



The symptoms of dehydration become increasingly severe with greater levels of dehydration.



A body water loss of as little as 1-2%, considered mild dehydration, is shown to impair cognitive performance.


Dehydration is generally considered to have a negative effect on cognitive function, despite several studies reporting comparable findings between hydration states. 

Recommendations to avoid moderate dehydration (≥ 2% loss in body mass) because this deficit may impair optimal cognitive performance. 

To determine whether cognitive function is impaired by hypohydration, and investigate the existence of the proposed critical water deficit of ≥2% loss in body mass purported to diminish cognitive performance, we conducted a systematic search of the literature and examined appropriate studies by meta-analysis. 

Overall, cognitive performance was not found to be impaired by dehydration.



With aging  the thirst sensation diminishes.



Many senior citizens suffer symptoms of dehydration,which  contributes to morbidity in the elderly, especially during conditions that promote insensible free water losses, such as hot weather. 



The main symptom of dehydration in the elderly is fatigue.



Risk factors for dehydration include:  exerting oneself in hot and humid weather, habitation at high altitudes, endurance athletics, elderly adults, infants, children and people living with chronic illnesses, and medications.



The impaired response to thirst or inadequate ability to access free water in the face of excess free water losses are to the main causes of dehydration.



Free water leaves the body in two ways – sensible loss such as osmotic diuresis, sweating, vomiting and diarrhea, and insensible water loss, occurring mainly through the skin and respiratory tract. 



It is caused by a wide range of processes and diseases  that impair water homeostasis.



Dehydration occurs primarily through either impaired thirst/water access or sodium excess.



Dehydration occurs when water intake is not enough to replace free water loss.



Free water loss occurs due to normal physiologic processes, including: breathing, urination, perspiration, or diarrhea and vomiting. 



Dehydration can be life-threatening: seizures,respiratory arrest, and osmotic cerebral edema if rehydration is overly rapid.



Management: thirst is normally an adequate guide to maintain proper hydration.



Water intake requirements vary with weight, environment, diet and genetics: 


exercise, exposure to hot environments, or a decreased thirst response, additional water may be required. 



In athletes in competition drinking to thirst optimizes performance and safety, despite weight loss.



Whole-body sweat losses in men can exceed 2 L/h during competitive sport, with rates of 3–4 L/h observed during short-duration, high-intensity exercise in the heat.



When large amounts of water are being lost through perspiration, electrolytes, especially sodium, are also being lost.



Exercising and sweating for 4–5 hours with a sweat sodium concentration of less than 50 mmol/L, the total sodium lost is less than 10% of total body stores.



Total body sodium stores are approximately 2,500 mmol, or 58 g for a 70-kg person.



Exercise sweat  losses are well tolerated by most people. 


Recommended intake volumes in the elderly are the same as for younger adults (2.0 L/day for females and 2.5 L/day for males).






For  minor dehydration the most effective is drinking water and stopping fluid loss. 



Plain water restores only the volume of the blood plasma, and inhibs the thirst mechanism before solute levels can be replenished.



In some cases, replenishment of necessary water and electrolytes is through oral rehydration therapy or fluid replacement by intravenous therapy.



Oral rehydration is the treatment of choice for mild dehydration.



Intravenous rehydration must be isotonic or hypertonic.



Seawater or drinks with significant alcohol concentration will worsen the condition. 



For severe dehydration where fainting, unconsciousness, or other severely inhibiting symptom is present emergency attention is required. 



Fluids containing replacement electrolytes are given orally or intravenously with continuing assessment of electrolyte status.



In most cases dehydration resolved successfully.







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