What is an isotonic solution?

An isotonic solution is any external solution that has the same solute concentration and water concentration compared to body fluids. In an isotonic solution, no net movement of water will take place.

An isotonic solution has the same osmolarity, or solute concentration, as another solution. If these two solutions are separated by a semipermeable membrane, water will flow in equal parts out of each solution and into the other.

The effect is zero water flow between the two solutions, although water is moving both ways. In biology, some cells must be maintained in an isotonic solution to support cellular functions.

Many animal cells, which lack a cell wall to provide support against the effects of water pressure, rely on the stability of the external environment to maintain their shape.

Most animals maintain the pH and osmolarity of the fluids inside of their bodies to create isotonic solutions to bathe their cells in. This solution can carry nutrients and water, but only in proportions equal to that inside the cell.

What is isotonic Solution

Isotonic solution:

  • An isotonic solution is one with a salt concentration that is exactly equal to that of blood cells.
  • If the surrounding solution has the same salt concentration as the solution inside the cell, the surrounding solution is referred to as isotonic.
  • The term ‘isotonic’ refers to the tonicity that exists between two distinct areas or solutions.
  • If the concentration gradient between two solutions is effectively zero, they are said to be isotonic.
  • Water should not flow from one side of the semipermeable membrane to the other.
  • The osmotic pressure exerted on the semipermeable membrane by both fluids should be the same.

Examples of Isotonic Solutions

For a solution to be isotonic compared to bodily fluids, it must have the same or a very similar solute concentration. Because this would rarely happen naturally, most solutions we would consider to be isotonic are designed to be that way. Some examples of this include:

  • Isotonic energy drinks
  • Normal saline (0.9% NaCl solution)
  • Lactated Ringer’s solution
  • 5% dextrose in water (D5W)

Examples of Isotonic solutions in biology

Blood Cells

When the plasma surrounding blood cells is an isotonic solution, compared to the solution inside the blood cells, the cells function normally. The isotonic solution allows the cells to move water and nutrients in and out of the cells.

This is necessary for blood cells to perform their function of delivering oxygen and other nutrients to other parts of the body. If the cells are in a hypertonic environment, they will become plasmolyzed and will not contain enough water to perform cellular functions.

If the cells exist in a hypotonic environment, they will lyse, spilling their contents into the bloodstream. This can cause dangerous side effects, as well as the loss of many blood cells. These events can be seen in the graphic below.

osmotic conditions in red blood cells in human blood

To avoid either of the negative situations from happening during the transfusion of nutrients and medicine, the solution that carries the medicine must be isotonic, compared to the patient’s blood.

The osmolarity of the IV fluid can be adjusted using special salts and sugars that act simply as solutes to dilute or strengthen a substance.

Once a medicine is an isotonic solution compared to the blood, it can be added through an IV and no damage will occur to blood cells.

Osmoconformers and Osmoregulators

In nature, there are two types of organisms: those that conform to the osmolarity of the environment, and those that regulate the osmolarity of their body to be different from the environment.

The first are known as osmoconformers and have evolved to have cells that match the osmolarity of the environment. These animals always exist in an isotonic solution, because they have evolved to be the same concentration as the environment.

This condition can be seen in many of the “lower” forms of life such as the sea slugs, coral, and jellyfish. The other group, the osmoregulators, do not exist in an isotonic environment.

This means that water tends to want to enter or leave their bodies, and they have various methods for dealing with this. However, inside of an osmoregulator, the cells will still exist in an isotonic solution, as the organism needs its cells to remain functional.

Both osmoregulators and osmoconformers have different benefits for conducting life the way they do, but an isotonic solution is usually created around cells.

Medical use of Isotonic solutions

  • Oral Rehydration Therapy uses the isotonic solution for electrolyte supplementation and maintenance of hydration in cases like enteritis, diarrhea, etc.
  • Saline treatment for treating extreme dehydration and hypernatremia (a condition wherein the serum concentration of sodium is increased).
  • Saline solution is used as a vehicle for parenteral, especially intravenous, administration of drugs as saline solution and blood plasma are isotonic.
  • Lactated Ringer’s solution and Hartmann’s solution are isotonic with blood plasma and used for the treatment of hypovolemia (blood volume is reduced may be due to injury or any other reason) and acidosis (blood acidity increases).
  • Saline solution is used for treating rhinosinusitis.
  • Saline solution is used as a vehicle for drugs that are to be administered via nebulization.
  • For ophthalmic disorders

Non-medical use of isotonic solutions

  • Sports drinks for providing hydration and electrolyte supplementation
  • Phosphate buffer saline is also used as a vehicle for maintaining cell cultures during experimentation

Hypertonic Solution vs Isotonic Solution

When a solution has a higher solute concentration than the solution present across the semi-permeable membrane, it is known as a hypertonic solution.

As a result, when a cell is placed in a hypertonic solution, an osmotic pressure gradient forces the solvent to move out of the cell to attain equal solute concentration across a membrane (Figure 1). This is visualized as cell shrinkage.

Thus, when a cell is placed in a hypertonic solution, it leads to shrinkage of the cell known as plasmolysis. In such a scenario, the cell membrane acts as a semipermeable membrane and the solution having a higher solute concentration in comparison to the cytosolic concentration is known as hypertonic.

Hypotonic Solution vs Isotonic Solution

When a solution has a lower solute concentration than the solution present across the semipermeable membrane, it is known as a hypotonic solution.

As a result, when a cell is placed in a hypotonic solution, an osmotic pressure gradient forces the solvent to move into the cell to attain equal solute concentration across a membrane. This is visualized as cell swelling.

When a cell is placed in a solution having a lower solute concentration in comparison to cytosolic concentration, the gradient difference results in the movement of the solvent into the cell resulting in swelling and eventually, bursting or cytolysis of the cell.

Usually, this happens in the case of animal cells, which are without any cell walls. However, in the case of plant cells that have a cell wall and a central vacuole, the rigid cell wall protects the cell from bursting and the vacuoles take up the excess water pushing up the cell membrane against the cell wall. This phenomenon is known as turgor pressure.

Isotonic Muscle Contraction

Isotonic contraction definition: In physiology, when the muscles change in length of the muscles resulting in a movement without a change in the muscle tension then this movement of the muscle is known as isotonic muscle contraction.

On the other hand, muscles can cause a change in muscle tension without a change in the muscle dimension or movement via isometric muscle contractions.

These contractions are commonly seen in the muscles responsible for grip in the hand and forearm.

Isotonic muscle contraction can be further subdivided into:

Concentric contractions

During this contraction, the muscles shorten to generate force to overcome resistance. For eg: Weight lifting towards the shoulders involves concentric muscle contractions.

Eccentric contractions

During this contraction, the muscles increase in length to generate force to overcome resistance. These contractions can be voluntary as well as non-voluntary.

Isotonic exercise definition

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

Here, it is important to understand that isotonic muscles will exhibit the same muscle tone. Hence, isotonic exercise may not result from isotonic muscles.

For example, in bicep curls, a person may exert more unequal focus on the right or left biceps thus resulting in non-isotonic muscles. On the contrary, in push-ups, both sides of the bodywork equally, and hence they result from isotonic muscles.