Fluids - Osmolality vs Tonicity

#med #disease

Note: osmoles differ from moles in that they take into consideration dissociation of ions
- For instance 1 mol of NaCl = 2 osmol = 1 osmol of Na⁺ + 1osmol of Cl^-
Note: when water is the solvent, osmolarity and osmolality are equal given the 1kg H2O = 1L H2O

Osmolarity = concentration of solute (osmoles) per volume of solvent = osmol/L
Osmolality = concentration of solute (osmoles) per mass of solvent = osmol/Kg
- ===Normal osmolality of blood = 275-295 mOsm/kg ===
  - Smithline and Gardner Formula for estimating serum osmolality $S e r u m O s m = 2 * N a (m E q / L) + \frac{G l u c o s e (m g / d L)}{18} + \frac{B U N (m g / d L)}{2.8}$
- D5W = 5% dextrose in water = 50g dextrose / 1000g H2O
  - Dextrose molar mass = 180 g/mol
  - Thus $\frac{50 g d e x t r o s e}{1000 g H 2 O} * \frac{1 o s m o l d e x t r o s e}{180 g d e x t r o s e} = \frac{0.278 o s m o l d e x t r o s e}{1000 g H 2 O} = 278 m O s m / k g$
- NS = 0.9% Saline = 9g sodium chloride / 1000mg H2O
  - NaCl molar mass = 58.4 g/mol
  - Thus $\frac{9 g N a C l}{1000 g H 2 O} * \frac{1 o s m o l N a^{+} + 1 o s m o l C l^{-}}{58 g d e x t r o s e} = \frac{0.308 m o l N a C l}{1000 g H 2 O} = 308 m O s m / k g$
Tonicity = effective osmolality typical used to compare two different solutions separated by a semipermeable membrane at steady state.
- Effective osmols are those which are unable to cross the membrane; those which do cross will equalize across the membrane and not contribute significantly to a difference in osmolality across a membrane