osmolarity vs osmolality

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help! whats the difference and why do we care? or do we care? do i need to worry about it?

i understand that osmolarity is the concentration of solutes measured in volume in L out side the body

and

osmolality is the concentration of solutes measured in weight in kg inside the body

but how does it pertain to F & E?

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Osmolarity describes the amount of solutes per liter of body fluid. (I pulled out my old adult health book - Lemone and Burke - to double-check myself and it's mentioned ONCE in the index.)

Osmolality measures the number of solutes per kilo of water in the body. It is used to describe the concentration of body fluids. The osmolality of extracellular fluids is dependent on the Na+ concentrations in the body. (Normal is about 275-295 mOsm/kg.)

You can estimate it by doubling the serum sodium concentration.

You can use serum osmolality values to figure out how the patient is dehydrated - if it's due to straight H2O loss, osmolality is high (like in declining renal function or someone who's taken a bunch of diuretics). With isotonic fluid loss (like with burns or bleeding), osmolality stays pretty normal.

That's all I know and all I can find about it in my old text. I'll bet others have some more input for you.

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Specializes in ICU.

these terms are sometimes used as reference tools for describing types of fluids: hypertonic, hypotonic, isotonic.

I remember the solutions being defined by the osmolarity/osmolality.

Just my 2 cents

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Specializes in med/surg, telemetry, IV therapy, mgmt.

They are merely measurements of concentration. Here is a past thread on the subject:

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