What Are Isotonic Solutions?

A woman getting isotonic IV fluids.
Sports drinks are enhanced with electrolytes and sugars to replace those lost during strenuous exercise.
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  • Written By: G. Robinson
  • Edited By: C. Wilborn
  • Last Modified Date: 29 January 2015
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In the general sense, two solutions are isotonic when they contain the same amounts of solutes, or dissolved substances, and therefore have the same osmotic pressure. As commonly used in the medical field, though, isotonic solutions are solutions which have the same concentration of solute as the cells in the human body. A cell placed in an isotonic solution will neither gain nor lose water.

When two aqueous solutions of different concentrations or tonicities are separated by a semi-permeable membrane such as a cell wall, water will migrate from the less concentrated, or hypotonic, side to the more concentrated, or hypertonic, side in an attempt to bring both sides into equilibrium. This process is known as osmosis. The greater the difference in the two solutions' concentrations, the higher the osmotic pressure will be, and the quicker the osmotic transfer will be.

If a living cell, which has a significant concentration of solutes, is placed in a dish of pure water, osmotic pressure will cause water from the dish to migrate into the cell, causing the cell to swell and possibly rupture. Conversely, if the cell is placed in a dish of water that contains solute in a higher concentration than the cell, water will flow from the cell out into the solution, causing the cell to shrivel and die. If the cell is placed in a dish of isotonic solution, there will be no net movement of water.


It is the nature of osmosis that the identity of the solute doesn't matter. Thus, salts, sugars, and other soluble compounds are all effective at regulating osmotic pressure. All may be used to prepare isotonic solutions.

Tonicity is of critical importance in biology and medicine because of its effect on living cells. Cells will only grow in isotonic solutions, and any drug administered intravenously must be adjusted to be effectively isotonic with human blood. A 0.9% solution of sodium chloride is considered isotonic with blood, although in fact its osmotic pressure is actually slightly higher.

Sports drinks are beverages enhanced with electrolytes and sugars to replace those lost through strenuous exercise, and may be either isotonic, hypertonic, or hypotonic, depending on the manufacturer's formula. Isotonic solutions containing these substances are generally considered best for those engaged in normal athletic activity, while the hypertonic versions are geared more towards providing energy during sustained, high-endurance events. The hypotonic sports drinks are best for athletes who need the fluids without the energy boost of the added carbohydrates.



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Post 3

@donbri5: Isotonic is a balanced tonic, whereas hypotonic and hypertonic are unbalanced tonics.

Post 2

An example of an isotonic solution would be saline solution or ringers lactate. A daily saline soluton can be used to help clear up the sinus cavity. Generally, these types of solutions are used in the medical field.

Post 1

Very interesting. I will be taking biology this summer. I will definitely ask questions on this site. I see where the hypertonic solution is good for an athlete that needs electrolytes and the energy boost and the hypotonic solution is best for athletes that do not need a boost of added carbs. Is there such a thing as an isotonic solution without it being hypertonic or hypotonic?

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