Dehydration

In physiology and medicine, dehydration (hypohydration) is the excessive loss of body water, with an accompanying disruption of metabolic processes. It is literally the removal of water (ὕδωρ) from an object; however, in physiological terms, it entails a deficiency of fluid within an organism. Dehydration of skin and mucous membranes can be called medical dryness.

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). In reality however, it emerges that by "hyponatremic" and "eunatremic" dehydration these authors are actually referring to "hypovolemia" (see definition below). In humans, it is thought that the most commonly seen type of dehydration by far is isotonic (isonatraemic) dehydration, but this effectively refers to hypovolemia. "Dehydration", is thus a term that has loosely been used to mean loss of water, regardless of whether it is as water and solutes (mainly sodium) or free water. Those who refer to hypotonic dehydration therefore refer to solute loss and thus loss of intravascular volume but in the presence of exaggerated intravascular volume depletion for a given amount of total body water gain. It is true that neurological complications can occur in hypotonic and hypertonic states. The former can lead to seizures, while the latter can lead to osmotic cerebral edema upon rapid rehydration. It is thus important to distinguish "dehydration" from "hypovolemia" and maybe limit the term "dehydration" to states of "hypernatremia" and call all other usage "hypovolemia" as that would greatly facilitate management.