Dehydration

Dehydration
Nurses encourage a patient to drink an oral rehydration solution to reduce the combination of dehydration and hypovolemia he acquired from cholera. Cholera leads to GI loss of both excess free water (dehydration) and sodium (hence ECF volume depletion—hypovolemia).
Classification and external resources
Specialty	Critical care medicine
ICD-10	E86
ICD-9-CM	276.51
DiseasesDB	3520
MedlinePlus	000982
eMedicine	article/801012
MeSH	D003681
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In physiology, dehydration is a deficit of total body water, with an accompanying disruption of metabolic processes. Dehydration can also cause hypernatremia(high level of sodium ions in the blood) and is distinct from hypovolemia (loss of blood volume, particularly plasma).

Dehydration occurs when free water loss exceeds free water intake, usually due to exercise or disease, but also due to high environmental temperature. Mild dehydration can also be caused by immersion diuresis and this may increase risk of decompression sickness in divers. Most people can tolerate a three to four percent decrease in total body water without difficulty or adverse health effects. A five to eight percent decrease can cause fatigue and dizziness. Loss of over ten percent of total body water can cause physical and mental deterioration, accompanied by severe thirst. Death occurs at a loss of between fifteen and twenty-five percent of the body water. Mild dehydration is characterized by thirst and general discomfort and is usually resolved with oral rehydration.

Dehydration occurs when water intake is not enough to replace free water lost due to normal physiologic processes (e.g. breathing, urination or perspiration) and other causes (e.g. diarrhea or vomiting). Hypovolemia is a related condition specifically meaning a decrease in volume of blood plasma— not of total body water. Both (total body water and plasma volume) are regulated through independent mechanisms in humans and should not be conflated. Some authors have reported three types of dehydration based on serum sodium levels: hypotonic or hyponatremic (referring to this as primarily a loss of electrolytes, sodium in particular), hypertonic or hypernatremic (referring to this as primarily a loss of water), and isotonic or isonatremic (referring to this as equal loss of water and electrolytes). Indeed, in humans, it has been commonly thought that the most commonly seen type of dehydration (by far) is isotonic (isonatraemic) dehydration. This usage is incorrect and the terms isotonic, isonatremic, and eunatremic dehydration actually all refer to hypovolemia and should therefore be abandoned in favor of the latter. Hyponatremic dehydration cannot exist because by definition depletion of total body water can only lead to hypernatremia so this term actually refers to coexistence of two separate disorders - hyponatremia and hypovolemia and again the term dehydration must be avoided. A classic example of hyponatremia coexisting with hypovolemia is Addison's disease where cortisol deficiency leads to ADH excess and hyponatremia but mineralocorticoid deficiency simultaneously leads to sodium loss and hypovolemia. The latter subjects are not dehydrated, on the contrary they are over-hydrated (from free water retention due to ADH excess).

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