Details information on the different fluid compartments of the body, and how to safely prescribe fluids for patients. Goes into details about electrolytes, and how different fluids affect the body differently.
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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
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