isotonicity
Cells are like tiny water balloons whose walls let water pass but hold back salts and sugars. If the fluid around them is too dilute, water rushes in and they swell and burst; if it is too concentrated, water is drawn out and they shrivel. Isotonicity means a solution carries the same effective concentration of dissolved particles as the body's own fluids, so water neither rushes in nor out and the cells stay comfortably the same size.
For injectable and ophthalmic products, isotonicity is matched to blood and tears, which behave like a 0.9% solution of sodium chloride (about 290 milliosmoles per kilogram). A solution at this level is isotonic; a more dilute one is hypotonic and can swell and rupture red cells (haemolysis); a more concentrated one is hypertonic and can shrink them and cause pain or tissue damage at the injection site.
Formulators adjust tonicity by adding inert solutes — sodium chloride, dextrose, mannitol or glycerin — to bring a too-dilute drug solution up to the right osmotic strength. The amount needed is calculated, often using each ingredient's sodium chloride equivalent.
A useful distinction: tonicity is what a real cell membrane experiences, while osmolarity simply counts dissolved particles. A solute that crosses the membrane freely adds osmolarity but contributes nothing to tonicity, so the two are not always identical.
0.9% sodium chloride is called normal saline precisely because it is isotonic with blood; pure water injected into a vein would be dangerously hypotonic and burst red cells.
Match the body's osmotic strength and cells keep their shape; miss it and they swell or shrink.
Hypotonic solutions are the more dangerous direction for intravenous use because they cause haemolysis. Slightly hypertonic solutions are often tolerated, especially when given slowly into a large vein where they are quickly diluted by the bloodstream.