Osmolality and tonicity

Definitions

• Molality - the number of moles of a solute per kilogram of solvent
• Osmolality - number of osmoles of solute per kilogram of solvent
• Osmolarity - number of osmoles of solute per litre of solvent
• Tonicity - the effective osmolality of a solution (relative to plasma)
• Equivalent - one equivalent (eq) is 1 mole of an ionized substance divided by its valence
  * 1 mole of Ca2+ is 40g
  * 1 eq of Ca2+ is 20g

Osmolality

Does not change with temperature

ECF osmolality = 285-290 mOsm/kg

Tonicity

Some solutes can easily cross membranes and thus are ineffective at exerting an osmotic force across cell membranes
--> Tonicity doesn't include these ineffective solutes

The only 2 ineffective solutes that are in significant concentration
--> Urea and glucose

Other notes

Calculated osmolarity

Calculated osmolarity
= (1.86x[Na+]) + [glucose] + [urea] + 9

Osmolar gap

Osmolar gap
= osmolality - osmolarity

Main use of osmolar gap

• Screening test for detection of abnormal low MW solutes, e.g. ethanol, methanol, ethylene glycol (antifreeze)