Fluid Balance

  • 60% of weight is made up of water.
  • 2/3 of this is in the intracellular compartment
  • 1/3 is extracellular which comprises plasma, interstitial fluid and transcellular fluid (e.g. synovial, peritoneal and ocular).
  • The two types of pressures acting on this water are oncotic (power exerted by proteins to draw fluid back into the cell) and osmotic pressure (the ability of solute to attract water across a semipermeable membrane).
  • The main extracellular cation is sodium whilst potassium is the main intracellular ion.
  • Movement of fluid between the extracellular and intracellular compartments is dictated by osmotic pressure. Therefore isotonic fluids (have the same osmolality as plasma) will stay in extracellular compartment.
  • Oncotic pressure drives fluid into plasma from the interstitial fluid compartments. The other starling force is hydrostatic pressure which does the opposite.

  • Crystalloids do not have oncotic pressure and are useful as maintenance fluids. Examples include Normal saline, Hartmann’s and Ringer’s lactate.
  • Colloids contain larger molecules that exert oncotic pressure thus drawing fluid back into plasma. These are good for establishing haemodynamic stability and include haemaccel and dextran.
  • Minimum volume of urine = 0.5-1ml/kg/hour.
  • Maintenance requirements in a 70kg patient can be met with 3L of crystalloid, 90mM Sodium and 60mM potassium. These maybe given using 1L N Saline and 2L 5% dextrose with 20mM potassium in each bag.

Leave a Reply

Your email address will not be published. Required fields are marked *