Why must the saline drip given to a patient be 'isotonic' and not plain water?
Red blood cells sit in blood plasma of a certain solute concentration.
Red blood cells sit in blood plasma of a certain solute concentration. An isotonic saline (0.9% NaCl) matches it, so there's no net osmosis and the cells stay normal. Inject plain water (hypotonic) and water rushes into the cells until they burst (haemolysis); inject something too concentrated (hypertonic) and water leaves, shrivelling them (crenation). Isotonic keeps them safe.
Key point
Isotonic fluid = same osmotic concentration as blood, so cells neither swell nor shrink.
Common mistake
assuming pure water is the safest thing to inject.
Memory tip
match the body's concentration (isotonic) or you burst/shrivel the cells.
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