Importance of isotonicity in parenteral
Isotonicity means two solutions have same osmotic pressure across the semi-permeable membrane. A parenteral preparation is said to be isotonic if it has same osmotic pressure as that of blood plasma. A parenteral preparation should be isotonic otherwise it may cause crenation or haemolysis of the RBC.
When a hypertonic (Having high solute concentration) solution is injected then it causes shrinkage of RBC present in blood plasma leading to crenation of the RBC due to flow of fluid from RBC into the solution.
When hypotonic solution (having low solute concentration) is injected then it causes swelling of RBC leading to haemolysis of the RBC due to flow of fluid from solution into the RBC.
Various agents having property to maintain isotonicity are used in formulation of parenteral products like sodium chloride, dextrose, Lactose etc. 0.9% w/v solution of sodium chloride is used as isotonic preparation.