Fluid Compartments

To describe the composition and control of intracellular fluid ~and the mechanisms by which cells maintain their homeostasis and integrity~

On average, the human body is ~60% water. Distribution of water content can be divided conceptually into:

• Intracellular fluid
  Composes 2/3^rds of total body water. ICF is:
  • Not a contiguous fluid space
  • Useful as the composition of cellular contents is relatively uniform:
    • Potassium is the dominant intracellular cation
      Sodium concentrations are low.
    • The dominant anion is protein
      Chloride concentration is relatively low.
    • Low in magnesium
• Extracellular fluid
  Composes the remaining 1/3^rd of total body water, and is further divided into:
  • Intravascular fluid
    Composes ~20% of ECF. This refers solely to plasma volume (as the volume of blood from cellular components is ICF). The ICF is:
    • Vital for transporting nutrients, waste, and chemical messengers between the plasma and cells
  • Transcellular fluid
    Composes ~7% of ECF, and describes the volume of CSF, urine, synovial fluid, gastric secretions, and aqueous humor.
  • Interstitial fluid
    Composes the bulk of ECF volume, and describes the fluid that occupies the volume between cells.

Variations

Actual total body water content varies predominantly with fat content. This leads to differences concentrations in:

• Neonates
  ~75-80%.
• Elderly
  ~50% by the age of 60, due to increased adiposity.
• Women
  Typically ~55%.

Measuring Volumes of Fluid Compartments

All methods rely on the indicator-dilution method:

• A known amount (i.e. known volume of a known concentration) of indicator with affinity to a particular compartment is given and allowed to equilibrate
• The concentration of the indicator is then measured
• The difference between the measured concentration and the initial concentration is proportional to the volume of the compartment

Indicators used for calculation of:

• Plasma volume
  A colloid that will be retained in the vascular compartment; e.g. radio-labeled albumin.
• ECF volume
  A substance which can enter the interstitium but not cells; e.g. thiosulfate.
• Total body water
  A substance which can enter all compartments freely; e.g. heavy water ($^2H_2O$).
• ICF volume
  Can be measured by the difference between calculated ECF volume and TVW.

References

1. Brandis, K. Fluid Compartments. Anaesthesia MCQ.
2. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.