Body Fluid Compartments

TBW & Major Compartments - Our Inner Ocean

• Total Body Water (TBW) constitutes ~60% of body weight in adult males; it's lower in females and obese individuals due to higher adipose tissue.
• 📌 60-40-20 rule for body weight percentages:
  • Total Body Water (TBW): 60%
  • Intracellular Fluid (ICF): 40% (~2/3 of TBW)
  • Extracellular Fluid (ECF): 20% (~1/3 of TBW)

⭐ TBW percentage decreases with increasing age and body fat content.

ECF Subdivisions - Beyond the Cell

• Interstitial Fluid (ISF): ~11L (20% body weight). Fluid in tissue spaces, bathing cells; an ultrafiltrate of plasma.
• Plasma: ~3L (5% body weight). Non-cellular, protein-rich fluid component of blood.
• Transcellular Fluid: ~1-2L. Specialized fluids like CSF, intraocular, pleural, peritoneal, synovial.

  ⭐ Transcellular fluid includes CSF, intraocular, pleural, peritoneal, and synovial fluids, collectively representing a small but functionally important volume.

Fluid Composition - The Ion Lineup

• 📌 KICK: K+ InCell Kation; Na+ dominant outside.
• Distinct ionic compositions of Intracellular (ICF) and Extracellular (ECF) fluids are crucial. They maintain cell volume, osmotic balance, and generate resting membrane potentials essential for nerve and muscle function.
• The Na+/K+-ATPase pump actively maintains these gradients: pumps 3 Na+ out, 2 K+ in.

• Normal plasma osmolality: 280-295 mOsm/kg H2O. Calculated by: Osmolality $\approx 2 \times [\text{Na}^+]_p + [\text{Glucose}]_p/18 + [\text{BUN}]_p/2.8$

⭐ The Gibbs-Donnan effect results in a slightly higher concentration of diffusible anions in the interstitial fluid compared to plasma due to the non-diffusible plasma proteins.

Measuring Compartments - Volume Voyagers

• Indicator Dilution Principle:
  • Method: Inject known quantity (Q) of marker, allow equilibration, measure concentration (C).
  • Formula: $V = Q/C$ (V=Volume of compartment, Q=Total quantity of marker injected, C=Concentration of marker in compartment).
• Markers 📌:
  • Total Body Water (TBW): D2O, Antipyrine.
  • Extracellular Fluid (ECF): Inulin, Mannitol, Sucrose.
  • Plasma Volume: RISA (Radio-Iodinated Serum Albumin), Evans Blue.
• Derived Compartments Calculation:
  • Intracellular Fluid (ICF) = TBW - ECF.
  • Interstitial Fluid (ISF) = ECF - Plasma Volume.

⭐ Inulin is a polysaccharide that is freely filtered by the glomerulus but not reabsorbed or secreted by the renal tubules, making it an ideal marker for ECF volume.

Fluid Shifts & Swells - Pressure Points

• Fluid movement across capillaries: governed by Starling forces.
• Starling Equation: $J_v = K_f [ (P_c - P_i) - \sigma (\pi_c - \pi_i) ]$
  • $P_c$: Capillary hydrostatic pressure (favors filtration)
  • $P_i$: Interstitial hydrostatic pressure (opposes filtration)
  • $\pi_c$: Capillary oncotic pressure (opposes filtration, due to plasma proteins like albumin)
  • $\pi_i$: Interstitial oncotic pressure (favors filtration)
• Edema: Palpable swelling from ↑ interstitial fluid volume.

⭐ A significant decrease in plasma albumin concentration (hypoalbuminemia) reduces plasma oncotic pressure ($\pi_c$), leading to increased fluid filtration into the interstitium and edema.

High‑Yield Points - ⚡ Biggest Takeaways

• Total Body Water (TBW) is ~60% of adult body weight; ICF constitutes 2/3, ECF 1/3.
• ECF includes Interstitial Fluid (ISF) (75% of ECF) and Plasma (25% of ECF).
• Major cations: K+ predominates in ICF; Na+ predominates in ECF.
• Osmolality is kept nearly identical in all body fluid compartments.
• The indicator dilution principle (V = Q/C) is crucial for measuring fluid volumes.
• Markers: D2O/3H2O (TBW); Inulin/Mannitol (ECF); Evans blue/RISA (Plasma).