tonicity

Tonicity is what an injection or eye drop actually feels like to your cells: will it make them swell, shrink, or stay just right? It compares a solution to the fluid inside the body, but it counts only those solutes that cannot freely cross the cell membrane. If a solute slips through the membrane easily, it cannot pull water in or out and so does not contribute to tonicity, even though it adds to total osmotic pressure.

Three labels are used. An isotonic solution matches body fluid, so cells neither gain nor lose water normal saline at 0.9 percent sodium chloride is the classic example. A hypotonic solution is too dilute, so water rushes into cells and red cells can burst (haemolysis). A hypertonic solution is too concentrated, so water leaves cells and they shrivel (crenation). Pharmacists adjust tonicity by adding inert agents such as sodium chloride or dextrose.

Tonicity and osmotic pressure are related but not identical. Osmotic pressure counts every dissolved particle; tonicity counts only the membrane-impermeant ones and is judged against living cells. This is why a urea solution can have a high osmotic pressure yet be effectively hypotonic, because urea crosses cell membranes freely.