Body fluid compartments

Body Fluid Compartments - The Body's Inner Ocean

• Total Body Water (TBW): Constitutes ~60% of adult male body weight. Formula: $TBW = 0.6 × Body Weight (kg)$.
• Intracellular Fluid (ICF): 2/3 of TBW (~40% body weight). Rich in K+, Mg2+, and phosphates.
• Extracellular Fluid (ECF): 1/3 of TBW (~20% body weight). Rich in Na+, Cl-, and HCO3-.
  • Interstitial Fluid (ISF): 3/4 of ECF.
  • Plasma: 1/4 of ECF.

⭐ Third Spacing: In states like sepsis or pancreatitis, fluid moves from the intravascular compartment to the interstitial or non-functional "third space" (e.g., peritoneal cavity), causing intravascular depletion and edema despite normal or even excess TBW.

Compartment Composition - What's in the Soup?

• Overall Osmolarity is Equal (~290 mOsm/L): Despite different compositions, water moves freely to maintain osmotic equilibrium.

⭐ Gibbs-Donnan Effect: Negatively charged proteins in the ICF repel Cl- ions, leading to a lower intracellular Cl- concentration than predicted by the Nernst potential alone. This also influences the distribution of other permeable ions.

Measuring Fluid Volumes - Tag, Dilute, Measure

The indicator dilution principle is used to measure fluid compartments. A known quantity of a tracer is injected, allowed to equilibrate, and its final concentration is measured.

• Principle: $Volume = \frac{Amount\ of\ tracer\ injected\ (Q)}{Final\ tracer\ concentration\ (C)}$

• Tracers for Direct Measurement:

  • Total Body Water (TBW): D₂O, ³H₂O, antipyrine.
  • Extracellular Fluid (ECF): Inulin, mannitol, sucrose (cannot cross cell membranes).
  • Plasma Volume (PV): Radiolabeled albumin (e.g., ¹²⁵I-albumin), Evans blue dye.

• Calculated Compartments:

  • Intracellular Fluid (ICF): $ICF = TBW - ECF$
  • Interstitial Fluid (ISF): $ISF = ECF - PV$

⭐ The Intracellular Fluid (ICF) volume, the largest compartment, cannot be measured directly. It must be calculated by subtracting the ECF from the TBW.

Fluid Dynamics - The Great Exchange

• Starling Equation: Governs fluid movement across capillaries, balancing hydrostatic and oncotic pressures.
  • $J_v = K_f [ (P_c - P_i) - \sigma (\pi_c - \pi_i) ]$
• Hydrostatic Pressure (P): Pushes fluid OUT of the capillary. Primarily driven by blood pressure.
• Oncotic Pressure (π): Pulls fluid IN, maintained by plasma proteins (mainly albumin).

• Filtration vs. Reabsorption:
  • Arterial end: Net filtration (hydrostatic > oncotic).
  • Venous end: Net reabsorption (oncotic > hydrostatic).

⭐ Hypoalbuminemia (e.g., in liver failure, nephrotic syndrome) ↓ plasma oncotic pressure, leading to generalized edema.

High‑Yield Points - ⚡ Biggest Takeaways

• Total Body Water (TBW) is approximately 60% of body weight, split between Intracellular Fluid (ICF) at 2/3 and Extracellular Fluid (ECF) at 1/3.
• The ECF is further divided into interstitial fluid (75%) and plasma (25%).
• Sodium (Na+) is the principal extracellular cation; Potassium (K+) is the main intracellular cation.
• Water shifts between compartments are driven primarily by osmotic gradients.
• ADH regulates plasma osmolality (water balance), while aldosterone regulates volume (sodium balance).