"Serum osmolarity" vs. "Serum osmolality"

[Jedi of Zen]

I'm trying to figure out the difference in definitions between serum osmolarity and serum osmolality.

My textbook (Fundamentals of Nursing, 6th edition, by Potter & Perry) states that normal serum osmolaRity in the blood is 280 - 295 mOsm/kg.

However, every website that I have ever looked up on the matter states that serum osmolaLity is 280 - 295 mOsm/kg.

Does that mean that the 2 terms are exactly synonymous? If not, then which is wrong - my book or all of those other websites?

Thanks!

[Music in My Heart]

Strictly speaking osmolarity and osmolality are not the same thing. The former is the osmoles of solute per liter of solution while the latter is the osmoles of solute per kilogram of solvent.

For dilute aqueous solutions, they are essentially equivalent. I suspect that's the source of the differences that you're observing.

[Daytonite, BSN, RN]

per norma m. metheny's book fluid & electrolyte balance: nursing considerations, 4th edition, page 6-7. . .

• osmolality - the number of osmoles per kilogram of water; tonicity is sometimes used instead of this term
  
• osmolarity - the number of osmoles per liter of solution

since this differs with what the other poster has told you i reviewed the information in this book very carefully to make sure i copied it correctly.

[Music in My Heart]

since this differs with what the other poster has told you i reviewed the information in this book very carefully to make sure i copied it correctly.

i guess i'm missing something. our replies look similar to me except that i generalized osmolality to the mass of any solute instead of specifying water.

i'm overstressed and under-rested so i guess i'm missing something obvious.

[Jedi of Zen]

per norma m. metheny's book fluid & electrolyte balance: nursing considerations, 4th edition, page 6-7. . .

• osmolality - the number of osmoles per kilogram of water; tonicity is sometimes used instead of this term
  
• osmolarity - the number of osmoles per liter of solution

since this differs with what the other poster has told you i reviewed the information in this book very carefully to make sure i copied it correctly.

the definitions that my professor gives are the same, except he uses "the concentration of solute" instead of "the number of osmoles".

i guess now i'm wondering what the difference is between a solute and an osmole...

[Jedi of Zen]

For dilute aqueous solutions, they are essentially equivalent. I suspect that's the source of the differences that you're observing.

Okay. So blood is a dilute aqueous solution, then?

[Daytonite, BSN, RN]

solute - a substance that is dissolved in a solution

osmole - the standard unit of osmotic pressure based on a one molal (one mole of solute per kilogram of solvent) concentration of an ion in a solution

you really need to get yourself a medical dictionary. all these terms were found in my copy of taber's cyclopedic medical dictionary.

[caliotter3]

I've always had problems understanding these concepts and probably won't understand them on my deathbed. I hope I don't need to be able to explain them to St. Peter. If that's the case, then I'm lost.

[Jedi of Zen]

solute - a substance that is dissolved in a solution

osmole - the standard unit of osmotic pressure based on a one molal (one mole of solute per kilogram of solvent) concentration of an ion in a solution

you really need to get yourself a medical dictionary. all these terms were found in my copy of taber's cyclopedic medical dictionary.

i have 2 medical dictionaries, thank you. i've come across those definitions already. my question was more about why the the terms "concentration of solute" and "number of osmoles" are used so interchangeably. my prof gave me a more elaborate answer today in class, however.

i'll try to stick strictly to my taber's next time, though. thanks.

[Daytonite, BSN, RN]

My question was more about why the the terms "concentration of solute" and "number of osmoles" are used so interchangeably.

You wanted to know the difference between their definitions and if the two terms were synonymous. Based on the answers you've gotten, clearly, the answer is "no".

[Jedi of Zen]

You wanted to know the difference between their definitions and if the two terms were synonymous. Based on the answers you've gotten, clearly, the answer is "no".

So why are the terms used interchangeably?

[EricJRN, MSN, RN]

In the relatively dilute environment of blood, they are essentially interchangeable.