Body Fluid Compartments

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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)

Body Fluid Compartments: Intracellular vs. Extracellular

⭐ 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. Body Fluid Compartments: ICF, Plasma, and Interstitial Fluid

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.
IonECF (mEq/L) (Plasma)ICF (mEq/L)Predominant
Na+135-14510-15ECF
K+3.5-5.0140-150ICF
Ca2+ (ionized)2.2-2.6<0.001 (free)ECF
Mg2+1.5-2.0~35 (total); ~1 (free)ICF
Cl-98-1064-10ECF
HCO3-22-288-12ECF
PO43- & Org. Anions~4~100ICF
Proteins (anionic)~16 (Plasma), ~2 (ISF)~55ICF > Plasma
  • 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.

Indicator dilution principle for body fluid measurement

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.

Starling forces and fluid movement at the capillary

⭐ 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).

Practice Questions: Body Fluid Compartments

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Flashcards: Body Fluid Compartments

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