It is important to realize the difference between HYPOVOLEMIA and DEHYDRATION, two terms which are often used interchangeably but in fact reflect different things. Read the Up-To-Date entry « Dehydration is not synonymous with hypovolemia » by Burton Rose if there is any confusion about this. HYPOVOLEMIA refers to any condition in which the extracellular fluid volume is reduced, and results in decreased tissue perfusion. It can be produced by either salt and water loss (e.g. with vomiting, diarrhea, diuretics, or 3rd spacing) OR by water loss alone, which is termed DEHYDRATION. Salt + water loss comes primarily from the extracellular fluid whereas pure water loss (dehydration) come from the total body water, only about 1/3 of which is extracellular. Thus for dehydration to produce the same degree of extracellular volume depletion as salt+water loss, 2.5 times as much fluid needs to be lost. Patients with DEHYDRATION are always hypernatremic.