The Osmosis Unit

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Plasma osmolality

Normal human plasma has an osmolality in the range of 285-295 miliosmol/kg. Agents which have an osmolality higher than 600 cause crenation (shrivelling up) of red blood cells and significant pain. Solutions which have an osmolality less than about 150milliosml/kg cause haemolysis (rupture of the red blood cells) and pain at the site of injection. Massive haemolysis can be fatal. Plasma osmolality is tightly controlled by homeostatic mechanisms. Changes in plasma osmolality are detected by osmoreceptors in the circulatory system. If the osmolality is too low (ie the blood is too dilute) the secretion of antidiuretic hormone (ADH)  is switched off, and the  blood slowly concentrates as water is excreted into the urine. Dehydration causes ADH to be switched on water conserved. A change of just 3 milliosmol in plasma osmolality causes a change from minimal to maximal ADH output.


IN order to avoid crenation or haemolysis injections and infusions should have an osmolality as close to plasma as possible. A solution that has the same osmotic pressure as another is called isotonic. In physiology isotonic generally assumes that a solution will have the same osmolality as blood. 

Large volume infusions should have an osmolality as close to 287- 290 milliosmol/kg  All injections should have an osmolality as close to the normal range as possible (285-295 milliosmol/kg). This is usually the case, however, some intramuscular injections have a very high osmolality and not surprisingly can cause pain at he site of the injection. 


What is the difference between osmolality and COP

Factors that affect osmolality

Osmolality measurement



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Last modified: 07/05/06