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Dehydration, refers to a deficit of total body water,〔Mange K, Matsuura D, Cizman B; et al. (1997). "Language guiding therapy: the case of dehydration versus volume depletion". Ann. Intern. Med. 127 (9): 848–53. PMID 9382413.〕 with an accompanying disruption of metabolic processes. It is also the reason for hypernatremia. The term ''dehydration'' must be distinguished from hypovolemia (loss of blood volume, particularly plasma). Dehydration occurs when free water loss exceeds free water intake, usually due to exercise or disease. Most people can tolerate a three to four percent decrease in total body water without difficulty. A five to eight percent decrease can cause fatigue and dizziness. Over ten percent can cause physical and mental deterioration, accompanied by severe thirst. A decrease more than fifteen to twenty-five percent of the body water is invariably fatal.〔Ashcroft F, Life Without Water in Life at the Extremes. Berkeley and Los Angeles, 2000, 134-138.〕 Mild dehydration is characterised by thirst and general discomfort and usually resolves with oral rehydration. == Definition == Dehydration occurs when water intake is insufficient to replace free water lost due to normal physiologic processes (''e.g.'' breathing or urination) 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 term ''isotonic or isonatremic or 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〔Hooper L, Abdelhamid A, Ali A, Bunn DK, Jennings A, John WG, Kerry S, Lindner G, Pfortmueller CA, Sjöstrand F, Walsh NP, Fairweather-Tait SJ, Potter JF, Hunter PR, Shepstone L. Diagnostic accuracy of calculated serum osmolarity to predict dehydration in older people: adding value to pathology laboratory reports. BMJ Open. 2015 Oct 21;5(10):e008846. . PubMed PMID 26490100.〕〔Sam R, Feizi I. Understanding hypernatremia. Am J Nephrol. 2012;36(1):97-104. . Epub 2012 Jun 27. Review. PubMed PMID 22739333. 〕 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). ''Dehydration'' is thus a term that has been very loosely used to either mean ''true'' dehydration or as a proxy for hypovolemia and only the former is the proper use of this term.〔 This is important〔Bhave G, Neilson EG. Volume depletion versus dehydration: how understanding the difference can guide therapy. Am J Kidney Dis. 2011 Aug;58(2):302-9. . Epub 2011 Jun 25. PubMed PMID 21705120; PubMed Central PMCID: PMC4096820.〕 because total body water is not controlled via sodium regulation, only intravascular volume is so controlled and this distinction is important to guide therapy. Dehydration can be life-threatening when severe and lead to seizures or respiratory arrest, and also carries the risk of osmotic cerebral edema if rehydration is overtly rapid. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dehydration」の詳細全文を読む スポンサード リンク
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