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Fluid prescribing
Fluid compartments
There are two main fluid compartments:
Intracellular and extracellular
The extra cellular compartment is split
further into intravascular and interstitial.
The fluid distribution of a 70kg man
Approximately 65% (28L) of fluid is in the
intracellular fluid
Around 35% (14 litres) of fluid is in the
extracellular fluid
Starlings hypothesis → movement of water
across capillary wall is dependent on the Electrolyte distribution
balance between oncotic pressure gradient
and hydrostatic pressure gradient across
capillary
distribution of water across intra- and
extracellular compartments dependent on
extracellular sodium ions.
This gradient is maintained by the sodium-
potassium ATPase pump
Intracellular fluid:
High potassium concentration
Low sodium concentration
The solute concentrations remain more or
less constant
Fluid prescribing 1
, Extracellular fluid:
High sodium concentration
Low potassium concentration
Oncotic pressure (osmotic pressure):
This is determined by the presence of
large molecular weight particles (i.e
proteins ) in the blood.
In a healthy individual the protein
concentration should be MUCH lower in
the interstitial fluid
Hydrostatic pressure:
Hydrostatic pressure is determined by
circulatory pressure
Other pressures such as oedema,
mechanical restriction i.e infection, plasters
casts or bandaging influence hydrostatic
pressure too.
Maintenance fluids
Sources of fluid gain:
Oral fluids
Parenteral fluids
Water released from metabolism (not included in clinical calculations)
→ Healthy average adult requires 2 -2.5 litres of maintenance fluid daily (1.5 litres for
urine loss and 500-800ml insensible losses)
25-30ml/kg/day of water
1 mmol/kg/day of sodium, potassium and chloride
Fluid prescribing 2
Fluid prescribing
Fluid compartments
There are two main fluid compartments:
Intracellular and extracellular
The extra cellular compartment is split
further into intravascular and interstitial.
The fluid distribution of a 70kg man
Approximately 65% (28L) of fluid is in the
intracellular fluid
Around 35% (14 litres) of fluid is in the
extracellular fluid
Starlings hypothesis → movement of water
across capillary wall is dependent on the Electrolyte distribution
balance between oncotic pressure gradient
and hydrostatic pressure gradient across
capillary
distribution of water across intra- and
extracellular compartments dependent on
extracellular sodium ions.
This gradient is maintained by the sodium-
potassium ATPase pump
Intracellular fluid:
High potassium concentration
Low sodium concentration
The solute concentrations remain more or
less constant
Fluid prescribing 1
, Extracellular fluid:
High sodium concentration
Low potassium concentration
Oncotic pressure (osmotic pressure):
This is determined by the presence of
large molecular weight particles (i.e
proteins ) in the blood.
In a healthy individual the protein
concentration should be MUCH lower in
the interstitial fluid
Hydrostatic pressure:
Hydrostatic pressure is determined by
circulatory pressure
Other pressures such as oedema,
mechanical restriction i.e infection, plasters
casts or bandaging influence hydrostatic
pressure too.
Maintenance fluids
Sources of fluid gain:
Oral fluids
Parenteral fluids
Water released from metabolism (not included in clinical calculations)
→ Healthy average adult requires 2 -2.5 litres of maintenance fluid daily (1.5 litres for
urine loss and 500-800ml insensible losses)
25-30ml/kg/day of water
1 mmol/kg/day of sodium, potassium and chloride
Fluid prescribing 2
