Body Fluid Compartments and Composition

Body Fluid Compartments - Aqua Central

• Total Body Water (TBW): ~60% body weight (adult male), ~50% (adult female).
  • Varies with age, sex, adiposity.
• Rule of 60-40-20 (📌 Mnemonic: "Sixty, Forty, Twenty"):
  • TBW: 60% of body weight.
  • Intracellular Fluid (ICF): 40% of body weight (or 2/3 of TBW).
  • Extracellular Fluid (ECF): 20% of body weight (or 1/3 of TBW).
• ECF = Interstitial Fluid (ISF) (75% ECF) + Plasma (25% ECF).
• Transcellular fluid (CSF, synovial): small volume (~1L).

⭐ ICF constitutes about 2/3 of Total Body Water, making it the largest fluid compartment.

Body Fluid Compartments - The Great Divide

Total Body Water (TBW): ~60% body weight (adult male), ~50% (female). 📌 60-40-20 Rule: TBW ~60%, ICF ~40%, ECF ~20% of body weight.

• Intracellular Fluid (ICF): 2/3 TBW.
  • Major Cations: $K^+$, $Mg^{2+}$.
  • Major Anions: Proteins, organic phosphates (e.g., ATP), $PO_4^{3-}$.
• Extracellular Fluid (ECF): 1/3 TBW.
  • Major Cations: $Na^+$.
  • Major Anions: $Cl^-$, $HCO_3^-$.
  • Subdivisions:
    • Interstitial Fluid (ISF): ~80% ECF. Ultrafiltrate of plasma.
    • Plasma: ~20% ECF. Higher protein content than ISF.
    • Transcellular Fluid: Small % (CSF, synovial, intraocular).

⭐ Gibbs-Donnan effect: Non-diffusible plasma proteins cause unequal ion distribution between plasma & ISF, affecting osmotic pressure.

Body Fluid Compartments - Electrolyte Lineup

• ICF vs. ECF Composition: Distinct electrolyte profiles.
  • ICF: High $K^+$, $Mg^{2+}$, Phosphates ($HPO_4^{2-}$), Proteins. Low $Na^+$, $Cl^-$.
  • ECF: High $Na^+$, $Cl^-$, $HCO_3^-$. Low $K^+$. Plasma has more proteins than ISF.

• Gibbs-Donnan Effect:
  • Non-diffusible ions (e.g., plasma proteins) affect diffusible ion distribution across semipermeable membranes.
  • Maintains electroneutrality, causing slight ion shifts. E.g., plasma [$Cl^-$] < ISF [$Cl^-$].

⭐ The $Na^+$/$K^+$-ATPase pump actively maintains these ionic gradients, crucial for cell volume, excitability, and transport.

Body Fluid Compartments - Flow & Figure Out

• Total Body Water (TBW): ~60% body weight (male), ~50% (female).
  • Intracellular Fluid (ICF): 2/3 TBW. K⁺, HPO₄²⁻ rich.
  • Extracellular Fluid (ECF): 1/3 TBW. Na⁺, Cl⁻ rich.
    • Interstitial Fluid (ISF): ~75% of ECF.
    • Plasma: ~25% of ECF.
• Measurement (Indicator Dilution): $V = Q/C$
  • TBW: D₂O, HTO.
  • ECF: Inulin, Mannitol, Sucrose. (📌 In My Soup)
  • Plasma Volume: Evans blue, RISA.
  • Calculated: ICF = TBW - ECF; ISF = ECF - Plasma.
• Fluid Movement:
  • Osmosis: Water follows solutes.
  • Starling Forces (capillaries): $J_v = K_f [ (P_c - P_i) - \sigma (\pi_c - \pi_i) ]$
    • $P_c$ (capillary hydrostatic) pushes fluid out.
    • $\pi_c$ (capillary oncotic - albumin) pulls fluid in.

⭐ Plasma osmolality (normal: 280-295 mOsm/kg H₂O) is key for fluid balance, set by Na⁺, glucose, urea.

High‑Yield Points - ⚡ Biggest Takeaways

• Total Body Water (TBW) is ~60% of body weight; lower in females/obese.
• Intracellular Fluid (ICF) is 2/3 TBW; Extracellular Fluid (ECF) is 1/3 TBW.
• ECF = 75% Interstitial Fluid (ISF) + 25% Plasma.
• K+ is the main ICF cation; proteins & organic phosphates are main anions.
• Na+ is the main ECF cation; Cl- & HCO3- are main anions.
• Osmolality (~290 mOsm/kg H2O) is equal across all fluid compartments.
• Volume markers: Plasma volume (radiolabeled albumin), ECF volume (inulin, mannitol).