Where Is Water in Your Body? The Two-Compartment Map

60% of You Is Water—But Where?

Total body water (TBW) ≈ 60% of body weight (varies by age, sex, fat %). It's distributed in two main compartments separated by cell membranes.

1. Intracellular Fluid (ICF) — 2/3 of TBW (~40% body weight)

• Inside cells — cytosol, organelles, nucleus.
• High K⁺, Mg²⁺, phosphate, proteins.
• Site of metabolism: glycolysis, Krebs cycle, protein synthesis, DNA replication.

2. Extracellular Fluid (ECF) — 1/3 of TBW (~20% body weight)

Subdivided into:

• Interstitial Fluid (ISF) — ~3/4 of ECF: Bathes cells in tissues; lymph precursor; high Na⁺, Cl⁻, HCO₃⁻. The "internal environment" (Claude Bernard).
• Plasma — ~1/4 of ECF: Fluid portion of blood; carries cells, proteins (albumin, globulins), nutrients, hormones, waste. High Na⁺, Cl⁻, HCO₃⁻, proteins.
• Transcellular Fluid — small but specialized:
  • Cerebrospinal fluid (CNS cushion)
  • Digestive secretions (saliva, gastric, bile, pancreatic, intestinal)
  • Synovial fluid (joint lubrication)
  • Pleural, pericardial, peritoneal fluids (serous lubrication)
  • Aqueous & vitreous humor (eye)

Key Barriers & Flows

• Cell membrane: ICF ↔ ISF (selective, active transport, osmosis).
• Capillary endothelium: Plasma ↔ ISF (Starling forces: hydrostatic vs. oncotic pressure).
• Blood-brain barrier: Plasma ↔ CSF (tight junctions, selective).

Why It Matters

• Dehydration: Losses hit ECF first → hypovolemia → shock.
• Edema: ECF excess (heart failure, kidney disease, low albumin).
• IV Fluids: Crystalloids (saline, lactated Ringer's) expand ECF; colloids (albumin) stay in plasma.
• Drug Distribution: Hydrophilic drugs → ECF; lipophilic → ICF + fat.

Your body isn't a water balloon—it's a precisely regulated two-compartment hydraulic system.