• Crystalloids are solutions in water that dissolve easily and may form crystals


  • A particle with a large molecular weight (MW > 30,000 Da) that is suspended in a solution


  • A substance that dissociates into ions in solution


  • The diffusion of solvent molecules across a semi-permeable membrane into a region in which there is a higher concentration of a solute to which the membrane is impermeable

Osmotic Pressure

  • The pressure required to prevent solvent migration due to osmosis

Oncotic Pressure

  • The component of the total osmotic pressure that is due to colloids


  • A unit of osmotic pressure equivalent to the amount of solute that dissociates in solution to form one mole of particles (molecules and ions)
  • The molecular weight of a solute, in grams, divided by the number of ions or particles into which it dissociates in solution.


  • The number of osmoles per kilogram of solvent
  • (does not change with temperature)


  • The number of osmoles per litre of solvent
  • (changes with temperature)


  • The effective osmolality of a solution
  • The osmolality of a solution relative to plasma (hypotonic, eutonic, hypertonic)

Ineffective osmole

  • A substance that can easily cross the membrane separating two fluid compartments, rendering it ineffective in exerting an osmotic pressure

Hydrostatic Pressure

  • The pressure exerted by a fluid owing to its potential energy, ignoring its kinetic energy


  • The process by which fluid is forced through a membrane or other barrier because of a difference in pressures on the two sides


Fluid Compartments

The following compartment divisions is not the classic arrangement. The kinetically slow compartments of bone and dense connective tissue are part of the ECF. However, if find this arrangement simpler to remember.

For a fuller discussion, read the Brandis chapter "Distribution of Body Water"

Fluid Compartments.jpg