I3ii: Describe the measurement of body fluid compartments

Total Body Water = 42L

  • 60% of body weight (i.e. 70kg x 0.6 = TBW)
  • Varies with age (↑infant, ↓elderly)
  • Varies with muscle/fat content (↑fat ∴↓TBW)
  • Contained in 2 compartments, separated by a cell membrane
    1. ECF
    2. ICF

Measurement

  • Isotopically labelled H2O → deuterium (2H)
  • Accurate results
  • Reproducible within 2%

ECF = 23L

  • 55% of TBW

→ Interstitial fluid

→ Plasma

→ Bone & connective tissue fluid

→ Transcellular fluid

  • Bone, connective tissue & transcellular fluid is significant amount but not mobilised quickly
  • ∴Interstitial & plasma KA ‘FUNCTIONAL ECF’

Isotope Tracer

  • e. chloride isotopes
  • very small
  • Distribute throughout ECF but also may enter cell
  • ∴Overestimate

Crystalloid Tracer

  • e. inulin, mannitol
  • Less diffusible
  • ∴underestimate
  • If you wanted estimate of the ‘non-fluid ECF’ → you would need to wait >24hrs

Plasma (of ECF)

  • An actual real fluid collection
  • Where blood cells are suspended

Measurement

  • Evan’s Blue
  • Dye which binds albumin
  • But albumin is continuously lost to interstitial space
  • ∴multiple plasma samples are taken & graph [   ] vs time is extrapolated back to zero

Blood Volume

Calculated from known Plasma Vol & HCt

Blood Vol

Whole Blood

Labelled Red Cells → 51Cr

No measurement

  • ISF
  • ICF
  • Transcellular fluid