Fluid Management and Perturbations in Volume Status

Total body water (TBW): Water comprises approximately 60% of lean body weight in men and 50% in women. Two-thirds of TBW is intracellular fluid (ICF) and one-third is extracellular fluid (ECF). ECF is further subdivided into intravascular and interstitial spaces in a ratio of 1:4.
- Example: For a healthy 70-kg man:
  - ICF = 2/3 TBW = 0.66 × 42 = 28 L
  - ECF = 1/3 TBW = 0.33 × 42 =14 L
  - Intravascular compartment = 0.25 × 14 = 3.5 L
  - Interstitial compartment = 0.75 × 14 = 10.5 L
- The distribution of water between intravascular and interstitial spaces can be affected by changes to the Starling balance of forces. Low oncotic pressure (i.e., low albumin states) and high hydrostatic pressure (i.e., Na⁺-retentive states) increase the movement of fluid from vascular to interstitial compartments, which is an important step in the development of edema.
Total body Na⁺: 85%–90% of total body Na⁺ is extracellular and constitutes the predominate solute in the ECF. Changes to the body’s total Na⁺ content typically results from a loss or gain of this Na⁺-rich fluid, leading to contraction or expansion of the ECF space.

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