Tonicity is effective osmolality of a solution. Free water will try to equilibrize across a permeable membrane when it separates 2 solutions.
Normal range for serum osmolality: 275 to 295 mOsm per kg water
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Serum Osmolality
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Tonicity
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< 275
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hypotonic
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275 to 295
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normotonic
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> 295
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hypertonic
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When cells in the body are exposed to a solution they will react to any difference in the osmotic pressure.
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Tonicity
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Effect on RBCs
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hypotonic
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swell
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normotonic
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normal
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hypertonic
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shrink
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