effective osmolality question

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effective osmolality = 2 x [Na-]p + [glucose]/18

the glucose portion of this equation is explained

as follows (in my textbook):

"glucose concentration is divided by one tenth of its molecular weight to calculate the number of osmotically active particles per liter."

why does the equation use 18?

(it also explains that including the BUN is a more accurate estimate of the actual osmolality, but the measure of the efective plasma osmolaity without consideration of the BUN term is the more physiologically meaningful estimate - what does this really mean?)

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The equation uses 18 because as your definition says, the glucose concentration is divided by 1/10 of its molecular weight...which happens to be 18.

Glucose has a molecular wt. of 180.16 G Mol

1/10 of about 180 is 18.

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