Water-iones imbalance
The general clinical terms for fluid volume abnormalities are dehydration and overhydration. Both conditions are associated with a change in the extracellular fluid volume.
Dehydration is an ambiguous term that does not distinguish between simple water loss and loss of sodium. Both water loss and sodium loss are associated with a decrease in the extracellular fluid volume. A simple water deficit reduces the extracellular and intracellular fluid volume proportionally while a sodium chloride deficit always decreases only the extracellular fluid volume.
Clinical manifestations of dehydration include decreased skin turgor, soft and sunken eyeballs, dry mucous membranes, gray and cool skin. A reduction of plasma volume causes tachycardia, arterial hypotension and flat neck veins.
Classification of dehydration is based on osmotic pressure.
Isosmotic dehydration
Causes of isosmotic dehydration include hemorrhage, plasma exudation through burned skin, and gastrointestinal fluid loss. Initially, fluid is lost from the plasma and then is replenished from the interstitial space. No major change occurs in the osmolality of the extracellular fluid and therefore no water shifts into or out of the intracellular compartment.
Hyperosmotic dehydration
Hyperosmotic dehydration results from water deficits and is caused by decreased water intake or increased water loss. The latter can be induced by the excessive evaporation through skin in severe loss of sweat, because sweat is hypotonic, renal diseases or endocrine disorders such as diabetes mellitus or abnormal secretion of the antidiuretic hormone. Initially, fluid is lost from the blood plasma, which becomes hyperosmotic, causing a fluid shift from the interstitial fluid to the plasma. The rise in the interstitial fluid osmolality causes water to shift from the intracellular to extracellular compartment. Finally, the extracellular and intracellular fluid volumes both are decreased and the osmolality of both major fluid compartments is increased.