Fluid loss occurs with perspiration, respiration, urination, diarrhea, diseases causing abnormal fluid shifts and bleeding.

Fluids can be replaced orally, intravenously (IV), through a feeding tube, rectally, or by clysis (injection into the subcutaneous tissues).

Fluids given intravenously should have the same or nearly the same osmotic pressure (solute concentration) as that of the human blood and tissues, in which case the fluid is described as “physiologic”, “isotonic” or having “normal” salinity.  Examples of isotonic fluids commonly used in IV therapy are Normal Saline, D5W (5% dextrose in water) and Ringer’s Lactate.

Fluids having a solute concentration greater than normal are hypertonic, less than normal, hypotonic.

The danger of giving hypotonic fluids is their tendency to cause swelling and rupture of red blood cells (hemolysis) and swelling and rupture of tissue cells (tissue lysis) as well.   Also, there is a risk of excessive dilution of the extracellular fluid, resulting in a dangerous electrolyte imbalance  called hyponatremia.

The problem with giving hypertonic fluids is that the red cells might be shrunken and thus destroyed, and the patient is in danger of developing hypernatremia, a condition akin to dehydration.

In summary, IV fluids are given to replace lost bodily fluids.  IV fluids should be physiologic or isotonic to prevent red cell and tissue damage, and to avoid causing electrolyte imbalance.