The Characteristics and Uses of Isotonic Solution

Isotonic Solution: Characteristics and Uses

Water is a universal solvent and a basis for life. However, there is a fine balance of water that needs to be maintained for the cell to survive. BiologyWise helps you to understand why isotonic solutions are so important for the maintenance of life, and also talks about its uses in our lives.

The “Gift-Brunnen” is a spring that has pure water; drinking this water swells and bursts the epithelial cells lining the digestive tract, and it is, therefore, called the poison spring.

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A solution is simply a homogeneous mixture of a minor component (solute) and a major component (solvent), like sugar dissolved in water. Now, concentration of a solution can be the amount of sugar that is added to water to make a solution. Every system in nature tends to maintain a state in which the molecules are equally distributed in the space surrounding them. This may take place through the process of either diffusion or osmosis.

Osmosis occurs basically when two solutions are separated from each other through a selectively permeable membrane. There is movement of solvent molecules (and other small molecules) through the selectively permeable membrane from a region of its higher concentration to a region of its lower concentration till the concentration of the solvent molecule (and other small molecules) is equal on both sides of the selectively permeable membrane.


❖ The state in which the concentration of the solvent molecules is equal on both sides of the selectively permeable membrane is isotonicity, and the two solutions are said to be isotonic solutions.

❖ The net movement of solvent molecules across the semipermeable membrane is zero, which means equal number of solvent molecules move from one solution to the other.

Cell structure

❖ In biological systems, the cytoplasm of the cell is a solution made of water and other components like sugars, amino acids, certain proteins, vitamins etc., and it is separated from the surrounding interstitial fluid by a semipermeable cell membrane.

In order for the cell to survive, it has to maintain isotonicity with its surrounding, this is because:

  • If the cell is in a hypertonic (solution outside the cell is more concentrated) environment, solvent molecules will move out of the cell, and the cell will shrink.
  • If the cell is in a hypotonic (solution inside the cell is more dilute) environment, the solvent molecules will move into the cell, and the cell will swell and might even burst.
  • If the cell is an isotonic environment, there is no net flow of the solvent molecules across the cell membrane, and the cell maintains its integrity.

❖ When two different isotonic solutions, let’s say a sugar solution and a salt solution, are separated by a semipermeable membrane, there will be no net flow of solvent molecules. However, the solute molecules (sugar and salt) will lower their concentration gradient such that there is an equal concentration of the solute molecules across the semipermeable membrane.


Oral Rehydration Therapy

❖ Isotonic solutions are used in Oral Rehydration Therapy (ORT), which is a treatment for dehydration associated with gastroenteritis. ORT replenishes the body’s lost electrolytes and promotes the uptake of glucose and fluid by the intestinal epithelial cells.

Intravenous Isotonic Solutions

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❖ 0.9% NaCl solution (isotonic with blood) is sometimes given intravenously in case of severe dehydration and is used in the treatment of hypernatremia (a condition in which serum concentration of sodium is high) in individuals who have impaired water excretion. It can also be used as a medium for intravenous administration of drugs like propofol, which are not absorbed well by the intestinal lining.

❖ Lactated Ringer’s solution and Hartmann’s solution are special formulations that closely resemble blood plasma and are isotonic solutions. They are used in the treatment of conditions like hypovolemia (a condition in which blood volume is lowered either due to injury or other reasons) and acidosis (a condition in which the acidity of the blood increases).

Aqueous Base

Syringe and steroids

❖ When 0.9% NaCl solution is used for flushing the nasal cavity in addition to the routine medication used for rhinosinusitis, it is shown to have a beneficial effect.

❖ 0.9% NaCl or 0.14% H2CO3 solutions are used as mediums for dissolving drugs that are used for nebulization (administration of drugs in the form of aerosol), such as ipratropium or salbutamol.

❖ It can be used as an aqueous medium for making many medicines that are administered through the ocular routes such as rinse, eye drops, steroids, antihistamines, antibiotics, etc.

Other Uses

Energy drink

❖ Some sports drinks are isotonic solutions that hydrate the body, replenish the electrolytes that the body loses by sweating, and provide instant energy.

❖ Phosphate buffered saline (PBS) is isotonic to the body fluids and is used routinely as a medium for suspending live cells in biological research.

Thus, isotonicity is the basis of maintenance of life by maintaining the water balance that occurs spontaneously, and without any expenditure of energy from the cell.

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