What is the normal osmolality of body fluids?
The body fluid that we usually use to measure osmolality in humans is serum. Serum is what is left from blood once the cells and proteins have been removed. The particles that make up the osmolality of serum are glucose, sodium and potassium salts (Na+ and K+) and urea. The normal serum osmolality is 280-295 mOsm/kg. Often you will be asked for a sample of urine for urine osmolality testing too.
The serum osmolality is extremely close to the osmolality inside the cells that our body is made of. This is because the walls of our cells are permeable to the microscopic particles (ions and anions) and to water. Permeable means it allows these particles and water to move freely in and out through the cell walls. This keeps the osmolality the same on both sides of the cell walls throughout the body. This is important as the cells won't work properly if this doesn't happen.
Occasionally the osmolality of stools (faeces) may also be tested.
Why is osmolality measured?
Osmolality is measured when doctors are concerned that your fluid balance may have gone awry for some reason. This might be due to an illness or as a side-effect of medication. It may even be due to you having overloaded your system by drinking too much water.
They may also want to measure it to monitor the effects of medication designed to change the osmolality of your body tissues. Mannitol is used for this effect to reduce brain swelling if you have had a head injury or brain surgery.
Further reading and references
McCartney M; Waterlogged? BMJ. 2011 Jul 12343:d4280. doi: 10.1136/bmj.d4280.
Osmolality; Lab Tests Online
Hyponatraemia; NICE CKS, March 2015 (UK access only)
Hew-Butler T, Loi V, Pani A, et al; Exercise-Associated Hyponatremia: 2017 Update. Front Med (Lausanne). 2017 Mar 34:21. doi: 10.3389/fmed.2017.00021. eCollection 2017.
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