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Tonicity

For the sake of brevity we can use this space to define three related words which you will come across very frequently. These are hypotonic, hypertonic and isotonic. They refer to the osmotic pressure of a solution in relation to a physiological fluid (plasma or interstitial fluid). A solution which has the same osmotic pressure as a physiological fluid is said to be isotonic with plasma (etc). Normally we leave out the "with plasma" bit and just say "it's isotonic".

A solution which is hypotonic has an osmotic pressure below that of a physiological fluid and a hypertonic solution is above it. If you find yourself getting confused between hypo... and hyper... remember hypodermic (under skin).