Basic Concepts of Fluid and Electrolyte Therapy

Colloid – a fluid consisting of microscopic particles (e.g. starch or protein)
suspended in a crystalloid and used for intravascular volume
expansion (e.g. 6% hydroxyethyl starch, 4% succinylated gelatin, 20%
albumin, etc.).
Balanced crystalloid – a crystalloid containing electrolytes in a concentration
as close to plasma as possible (e.g. Ringer’s lactate, Hartmann’s
solution, Plasmalyte 148, Sterofundin, etc.).
Osmosis – this describes the process by which water moves across a
semi-permeable membrane (permeable to water but not to the substances
in solution) from a weaker to a stronger solution until the
concentration of solutes are equal on the two sides.
This force is termed osmotic pressure or, in the case of colloids e.g.
albumin, oncotic pressure. It is proportional to the number of atoms/
ions/molecules in solution and is expressed as mOsm/litre (osmolarity)
or mOsm/kg (osmolality) of solution. In clinical chemistry the term
‘osmolality’ is the one most often used. For example, out of approximately
280-290 mOsm/kg in extracellular fluid the largest single contributor
is sodium chloride. This dissociates in solution and therefore
its component parts Na + and Cl – exert osmotic pressure independ ently
i.e. Na + (140 mmol/kg), contributes 140 mOsm/kg, and Cl – (100 mmol/
kg) contributes 100 mOsm/kg. Additional balancing negative charges
come from bicarbonate (HCO 3 – ) and other anions. In the intracellular
space K + is the predominant cation (see below).
Because glucose does not dissociate in solution, each molecule,
although much larger than salt, behaves as a single entity in solution
and at a concentration of 5 mmol/kg, contributes only 5 mOsm/kg to
the total osmolality of plasma.
The cell membrane and the capillary membrane are both partially permeable
membranes although not strictly semi permeable in the
chemical sense (see below). They act, however, as partial barriers
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