Isotonic solutions are the ones that have the same solute concentration as that of another solution separated by a semi-permeable membrane so that there is no net movement of fluids from one solution to the other. Thus isotonic solutions maintain the same osmotic pressure.
- Blood Plasma
- Lacrimal fluid
- Cerebrospinal fluid
- Serous fluid
- 0.9% NaCl (Normal saline)
- Lactate Ringer’s solution
- 5% dextrose in water
- Ringer’s solution
The blood consists of the liquid part called Plasma in which the blood cells like RBC, WBC, and platelets are present. The concentration of ions, nutrients, and other particles inside the cell is the same as the nutrient and solute concentration of plasma. So water does not travel through the cell walls inside or outside the cells.
This is an important mechanism because if the isotonic condition is not maintained, depending upon solute concentration, water will either rush inside the cells and cause them to swell up and burst, or flow out of the cell and cause it to shrink. Thus the cells will die.
Lacrimal fluid is another isotonic solution example. It is the tear fluid secreted by the Lacrimal gland of the eye that forms an aqueous layer providing protection and nutrients to the conjunctiva and cornea of the eye. Lacrimal fluid maintains an alkaline pH of 7.4 and is isotonic with blood plasma as well as 0.9% NaCl. The electrolyte composition of the fluid is similar to blood plasma.
It is important that the fluid maintains the same osmotic pressure and ion concentration as that of the ocular cells of the eye. The secretion of the fluid is also driven by a change in osmotic pressure due to ion movement so that water flows through the gland to maintain the osmotic balance.
The cerebrospinal fluid is an extracellular biological fluid that circulates in the brain and spinal cord and is produced by the ependymal cells of brain ventricles. It has different electrolytic composition than blood plasma but is isotonic with it. CSF has very important functions in maintaining buoyancy, homeostasis, immunological protection, and waste clearance.
Because of the important functions being carried out by CSF, it is important that it remains isotonic with plasma and brain tissues. Due to its isotonic nature, it can function as a mechanical shock absorber and any change in pH or osmotic pressure of CSF can cause severe damage to the nervous system.
Serous fluid is secreted by the serous glands of the body and generally fills different body cavities. It resembles the plasma in composition and consists of water proteins and ions. Saliva and pericardial fluid are examples of serous fluids.
Since the serous fluid collects inside the body cavities it is important that they are isotonic to blood plasma. In case the serous fluids become hypertonic or hypotonic, then the cells comprising the cavities will be severely affected due to the abnormal flow of water through them.
0.9% NaCl (Normal saline)
0.9% NaCl contains 0.9 g Sodium chloride in 100 mL water (w/v). It is isotonic with blood plasma. This is because when red blood cells are placed in a 0.9% NaCl solution there is no net flow of water inside or outside the cells. Thus osmotic balance is maintained because the solute concentration of the RBC is equivalent to 0.9% NaCl solution. But if the cells are placed in a solution of higher strength of sodium chloride, there will be shrinkage and plasmolysis of the cells and a hypotonic solution will cause the cells to swell and burst.
Since Normal saline solution is isotonic with blood plasma, it is used in intravenous fluid transfer in patients. Normally IV saline is used to prevent dehydration during sickness and it expands the extracellular fluid volume without entering inside the cells and interfering with the normal cell composition. Normal saline has an osmolality of 308 mOsm/L.
Lactate Ringer’s solution
It is also used as a crystalloid isotonic intravenous fluid used in patients with low blood pressure. It consists of sodium chloride, sodium lactate, potassium chloride, and calcium chloride in water. It contains precursors of bicarbonate to prevent acidosis. It is also called Ringer’s Lactate or Hartmann solution. It has a pH 6.5 and osmolality of 273 mOsm/ L.
It resembles plasma electrolyte composition and thus is isotonic with it. This is why Ringer’s lactate solution is used to resuscitate body fluid after severe blood loss. Thus due to the same osmotic pressure of the solution as that of extracellular fluid, it can easily flow through the blood vessels without affecting the blood cells.
5% dextrose in water
Dextrose is a simple carbohydrate that is identical to Glucose. 5% Dextrose in water means 5 g of Dextrose dissolved in 100 mL of water (w/v) is isotonic to blood serum. This solution is used as an IV fluid because of its isotonic nature and also because it can supply similar energy to the body as Glucose.
It is a type of crystalloid IV fluid that is administered to patients with hypoglycemia (low blood sugar), insulin shock, and dehydration. It is also sometimes mixed with medicine as a diluent and supplied intravenously. After it is administered in the bloodstream, the dextrose is metabolized and then there is a free flow of water in the extracellular fluid (ECF).
Ringer’s solution is similar to the Lactated Ringer’s solution only without the lactate component. It consists of salts like sodium chloride, potassium chloride, calcium chloride, and sodium bicarbonate and sometimes minerals are also added. This makes it isotonic with serum because of the same electrolyte composition and thus does not interfere with the intracellular composition of body cells.
Ringer’s solution is used in laboratory experiments involving tissues and organs to maintain osmotic balance. It is also used clinically as an intravenous fluid to supply medicines in the treatment of patients.
Isotonic solutions are very biologically significant because they maintain the same solute concentration as that of cells in the body. So there is no unusual flow of fluid regarding the cells, thus causing no damage. This is why isotonic solutions are preferred as IV fluids because they do not disrupt the normal composition of cells and perform similar functions to that of other biological fluids like plasma, tear, saliva, etc.