Question

when is it approprirae to use hyper/ hypo/ isotonic solution?

Answer 1

Hypertonic solution:

Hypertonic solution has higher osmolarity than that of the body fluids. Thus, if a hypertonic solution is administered, water will move out of cell from lower to higher solute concentration, with the plasma membrane acting as the semi permeable membrane.

If a hypertonic solution is used as I.V. fluid, then the cells will shrink and water will come out from the cells to dilute the infused fluid. Hypertonic solutions are therefore used medically to correct conditions like electrolyte imbalance or unusual pH in body.

For example, 3% and 5% NaCl solutions are hypertonic solutions used in intravenous fluids to replenish electrolytes in the body. It is most commonly given to treat severe sodium deficiency or severe dilution hypoatremia (deficiency of Na+). These solutions must be administered slowly in order to prevent rupture of blood vessels.

Hypertonic solutions is also used to treat edema. Edema is the swelling of cells due to high amount of ions in the cell. Adding hypertonic solution would draw out the excess water and excrete it via urination.

Hypotonic solution:

Hypotonic solutions have an osmolarity lower than that of the body fluids. In presence of a hypotonic fluid therefore, water will move inside the cell to dilute the cytosol and maintain external and internal concentration equilibrium.

Thus if a hypotonic solution is used as I.V. fluid, the cells will swell as water would rush in the cell to maintain osmolar equilibrium. Hypotonic solution is therefore used to treat conditions which require replacement of intercellular fluid.

0.45% or 0.25% NaCl solution are commonly used as hypotonic solutions. As this fluid moves into the cell, it is used to replinish intracellular fluid. Such solutions are used to treat cellular dehydration.

It can also dilute the blood in case of high osmolarity, and therefore restore the electrolyte balance. Hypotonic solutions are used to treat for example, diabetic ketocidosis, which is a condition where there is a high concentration of ketone bodies in blood due to the lack of insulin.

Hypotonic solutions are used to treat hyperantremia in blood (excess Na+) as well as hypertonic dehydration.

Isotonic solution:

Isotonic solutions are those which have osmolarity same as that of the body fluids. Thus if such a fluid is administered, the blood volume will increase but there will be no change in the intercellular water concentration.

Isotonic solutions are usually 0.9% saline with or without 5% dextrose. Presence of dextrose makes the solution behave mildly hypotonic as dextrose is readily uptaken by cells as primary fuel source. Isotonic solutions only changes the extracellular fluid and does not cause movement between intracellular and extracellular compartments. Thus, this kind of solutions must be used when we want to change extracellular fluid composition.

For example, in order to administer blood products such as RBCs or platelets, isotonic solutions must be used as we want the infusate to stay in the blood and not move into cells. Similarly it can also be used to treat ionic imbalance in body fluids. It can also be used to treat mild hypoatremia.

Note: Normal blood osmolarity is 280-300 mM/L. Solutions below that are hypotonic and those above are hypertonic. Solutions in that range is isotonic.