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.