Isotonic Solution Definition

The term “isotonic” is used in physiology, anatomy as well as in physical chemistry. In physical chemistry, isotonic solutions are those that have equivalent osmotic pressure.

In physiology, isotonic solutions are the ones that have solute content equivalent to that mammalian blood. In anatomy, isotonic is a condition of muscle contraction wherein, the length of the muscle decreases under constant tension.

Isotonic solutions are the solutions with the same tonicity. The measure of relative solute concentration across a semipermeable membrane is known as tonicity. In other words, it is also the effective osmotic pressure gradient across a semipermeable membrane.

The tonicity of a solution determines the amount as well as the direction of the solvent movement across a semipermeable membrane. Tonicity or osmotic pressure gradient is generated due to the solute that is unable to cross the semipermeable membrane.

Types of Solution

Solutions can be classified into three types depending upon their tonicity:

Isotonic Solution
Hypertonic Solution
Hypotonic Solution

Isotonic is a descriptive word for isotonicity. At the cellular level, isotonicity is used to describe the property of a solution in which the concentration of solute is equivalent to the solute concentration of another solution.

Thus, isotonic solutions are those that have equal osmotic pressure and water potential as both the solutions have an equal concentration of water molecules. E.g., blood serum is isotonic to the physiological salt solution. Solutions with the same tonicity do not allow water to pass through the cell membrane.

What is Isotonic Solution?

In an isotonic solution, solute concentration is the same across the semipermeable membrane resulting in an equilibrium state. Due to the absence of a concentration gradient, there is no net movement of the solvent molecules across a semipermeable membrane that is zero. But this doesn’t mean that the solvent is inhibited from moving across the membrane. In reality, only the rate of solvent movement across the membrane is the same resulting in a net-zero movement.

Hypertonic Solution

Hypertonic solution is the solution that has a higher concentration of solute as compared to the solution across the semi-permeable membrane. If a cell is placed in a hypertonic solution, the solvent from the cell moves out into the solution due to an osmotic pressure gradient to attain an equal concentration of solute across the cell membrane. This results in cell shrinkage also known as plasmolysis.

The cell membrane in this scenario is a semipermeable membrane and the solution outside the cell is termed hypertonic because it has higher solute as compared to cytosolic concentration. In animal cells, the equivalent condition as the plant plasmolysis is crenation.

Hypotonic Solution

Hypotonic solution is the solution that has a lower concentration of solute as compared to the solution across the semi-permeable membrane. If a cell is placed in a hypotonic solution, the solvent from the cell moves into the solution due to an osmotic pressure gradient to attain an equal concentration of solute across the cell membrane. This results in cell swelling.

The solution outside the cell is termed hypertonic because it has a lower solute concentration as compared to cytosolic concentration. Cell swelling can eventually lead to bursting or cytolysis of cells. This usually occurs in animal cells as they do not have a cell wall.

However, the rigid cell wall of plant cells protects them from bursting, and the excess water is taken up by the large vacuoles pushing the cell membrane against the cell wall. This is known as turgor pressure.

Isotonic Solution Examples

Normal saline solution which is an aqueous solution of sodium chloride i.e., 9 gm/l of NaCl solution, is both iso-osmolar and isotonic to the blood plasma. It is also called a physiological solution due to similar sodium concentrations in blood.

Due to its isotonicity with blood plasma, the saline solution is used in medicine both topically as well as parenterally (i.e., injecting into the bloodstream), like in cleaning wounds, fluid replacement therapy, hydration maintenance, etc. Due to isotonicity, the red blood cells retain their shape in an isotonic saline solution.

If RBCs are placed 2 % (w/v) NaCl solution, RBCs will shrink (crenation) because 2 % (w/v) NaCl concentration is greater than sodium content of RBC in other words 2 % NaCl is hypertonic solution in this case.

On other hand, if RBCs are placed in 0.1 % (w/v) NaCl solution, RBCs will swell and eventually burst to release hemoglobin (hemolysis). This will occur the RBCs are placed in a hypotonic solution.

Thus, the only isotonic saline solution should be administered into the bloodstream i.e., intravenously. A hypertonic- or hypotonic solution can’t be administered directly into the bloodstream.

Interestingly, non-isotonic solutions can be used to prepare formulation for subcutaneous and intramuscular injection because in such cases blood cells don’t come in direct contact with the injected formulation.

Ophthalmic solutions are isotonic to avoid ophthalmic irritation and pain. The lacrimal fluid, which is present in the eye is isotonic with normal saline. A hypotonic solution can cause congestion due to intrusion of fluid into the ocular tissues while a hypertonic solution can cause extrusion of fluid from the tissue.

Examples of isotonic solutions: normal saline, Oral rehydration solutions, phosphate buffer saline, and Hartmann’s solution, and Lactated Ringer’s solution.

Medical use of Isotonic Solutions

In Oral Rehydration Therapy, an isotonic solution is used for maintaining hydration and electrolyte supplementation in cases like enteritis, diarrhea, etc.
In Saline treatment, the isotonic solution is used for the treatment of extreme dehydration and hypernatremia (a condition this leads to an increase in serum concentration of sodium).
Saline is isotonic to blood plasma and therefore is used as a carrier for parenteral drug delivery, especially for intravenous administration. Lactated Ringer’s solution and Hartmann’s solution which are isotonic with blood plasma, are used for treating hypovolemia (reduced blood volume due to injury or any other reason) and acidosis (increase in blood acidity).
Saline solution is also used in the treatment of rhinosinusitis.
For ophthalmic disorders

Non-Medical Use of Isotonic Solutions

Sports drinks to hydrate the body and provide electrolyte supplementation
Phosphate buffer saline to maintain cell cultures during experimentation

Measurement of Tonicity

There are two methods to measure the tonicity of a solution:

• Hemolytic Methods: This method depends on the change in size and shape of the RBCs that are suspended in the test solution. The principle involves comparing the packed cell volume (PCV) of the RBCs in the test solution to the reference solution and depending upon the results they are categorized as isotonic if PCVtest = PCVref, hypertonic if PCVtest < PCVref), hypotonic if PCVtest > PCVref.

• Colligative Methods: Tonicity is measured using colligative properties like-

Molecular concentration
White-Vincent method
Sodium chloride equivalent value
Freezing point data

Isotonic Muscle Contraction

In physiology, when the muscles change in length but their muscle tension remains constant then this movement is known as isotonic muscle contraction (-‘iso’= same; ‘tone’= tension).

On the other hand, isometric muscle contraction refers to a change in muscle tension without a change in muscle dimension (-‘iso’= same; ‘metric’ = length). These contractions can be seen in the grasping muscles in the hand and forearm. Isotonic muscle contraction is further divided into:

• Concentric Contractions: The muscles shorten during this contraction to generate force to overcome resistance. E.g: Weight lifting towards the shoulders

• Eccentric Contractions: The muscles increase in length during this contraction to generate force to overcome resistance. These can be a voluntary or involuntary contractions.

At the tissue level, isotonicity is called a state in which a muscle remains in a relatively constant state of tension, and its length changes. To be more specific, the term isotonic muscle refers to a muscular contraction wherein the muscle remains in relatively constant tension as its length changes.

What is Isotonic Exercise?

Exercises like bicep curls and push-ups, which involve a lifting and a lowering phase, are considered isotonic exercises. Bicep curls involve raising and lowering the arms while push-up exercise involves raising the lowering the body in a plank position.

Isotonic muscles exhibit the same muscle tone. Hence, isotonic exercise may not result in isotonic muscles. E.g., In bicep curls, if a person exerts unequal focus on right/left biceps it results in non-isotonic muscles. On contrary, both the sides of the body work equally in push-ups and hence they result in isotonic muscles.