I got a requisition for serum and urine osmolality. I’ve done some searching, but I still don’t understand what those mean. Help?
In: Biology
Osmolality, as you might guess, is related to the concept of *osmolarity* and, by extension, osmosis and molarity.
Molarity and osmolarity are similar, but *os*molarity brings in the concept of osmotic pressure by accounting for things that dissociate in solution. Salt, for example, dissolves into Na^+ and Cl^- ions, and so when you consider a solution of salt, its *osmolarity* is higher than its *molarity.*
With me so far? Good. Here’s how osmola***r***ity and osmola***l***ity differ: Osmolarity is defined per unit *volume of solution,* while osmolality is defined per unit ***mass of solvent.***
Osmolality is the overall concentration of dissolved stuff in a liquid. Say if you dissolved 1 gmol of salt (NaCl) in 1L of water, then it would have an osmolality of 2 osmole (abbreviated Osm) per L (or kg, because density of water is 1kg/L) of water. The 2 comes from each salt molecule having two ions (Na+, Cl-).
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Basically, they’re just how concentrated your blood/urine are. Both being too concentrated and not concentrated enough can be signs of disease.
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