Fluid Shifts Between Compartments

Body Fluid Compartments - Our Inner Oceans

• Total Body Water (TBW): Approx. 60% of body weight in adult males (lower in females, obese individuals).
• Intracellular Fluid (ICF): Constitutes 2/3 of TBW.
  • Primary cation: $K^+$.
  • Primary anions: $PO_4^{3-}$, proteins.
• Extracellular Fluid (ECF): Constitutes 1/3 of TBW.
  • Primary cation: $Na^+$.
  • Primary anion: $Cl^-$.
  • Subdivisions:
    • Interstitial Fluid (ISF): ~75% of ECF.
    • Plasma: ~25% of ECF.
    • Transcellular fluid (e.g., CSF, synovial fluid; small volume).

⭐ The 60-40-20 rule is a key clinical approximation: 60% of total body weight is water, 40% is ICF, and 20% is ECF.

Starling's Forces - Capillary Pressure Play

• Fluid exchange between plasma & interstitium is governed by Starling's forces.

• Net Filtration Pressure (NFP) dictates movement: $NFP = (P_c - P_{if}) - (\mathbf{\pi}c - \mathbf{\pi}{if})$.

  • $P_c$: Capillary Hydrostatic Pressure; pushes fluid OUT (main outward force).
  • $\mathbf{\pi}_c$: Capillary Oncotic Pressure (albumin); pulls fluid IN (main inward force).
  • $P_{if}$: Interstitial Fluid Hydrostatic Pressure; pushes fluid IN.
  • $\mathbf{\pi}_{if}$: Interstitial Fluid Oncotic Pressure; pulls fluid OUT.

• Arteriolar end: Filtration favored ($P_c$ > $\mathbf{\pi}_c$).

• Venular end: Reabsorption favored ($\mathbf{\pi}_c$ > $P_c$).

  

⭐ Increased capillary permeability (e.g., inflammation) allows protein leakage into interstitium (↑$\mathbf{\pi}_{if}$), causing edema.

Osmolality & Tonicity - Solute's Mighty Pull

• Osmolality: Measure of total solute concentration per kg solvent.
  • Normal plasma: 280-295 mOsm/kg $H_2O$.
  • Calculated: $2 \times [\text{Na}^+] + \frac{[\text{Glucose}]}{18} + \frac{[\text{BUN}]}{2.8}$ (units: Na mEq/L, others mg/dL).
  • Mainly regulated by ADH.
• Tonicity: Effective osmolality; drives H₂O movement across membranes. Depends on impermeable solutes.
  • Isotonic: No net H₂O movement (e.g., 0.9% NaCl).
  • Hypotonic: H₂O enters cells → cells swell (e.g., 0.45% NaCl).
  • Hypertonic: H₂O leaves cells → cells shrink (e.g., 3% NaCl, Mannitol).
• 📌 Osmolality: all solutes. Tonicity: only non-penetrating solutes. Urea: ineffective osmole.

⭐ Mannitol (osmotic diuretic) ↑ ECF osmolality, draws H₂O from ICF. Used to ↓ intracranial & intraocular pressure.

Fluid Shift Scenarios - Clinical Case Clues

• Heart Failure: ↑$P_c$ (systemic/pulmonary) → Peripheral/pulmonary edema. Clues: Dyspnea, JVD, crackles.
• Liver Cirrhosis: ↓$\pi_p$ (↓albumin synthesis) + ↑$P_c$ (portal HTN) → Ascites, peripheral edema. Clues: Jaundice, spider nevi.
• Nephrotic Syndrome: ↓$\pi_p$ (proteinuria) → Generalized edema (anasarca), periorbital puffiness. Clues: Frothy urine, >3.5 g/day proteinuria.
• Sepsis/Burns: ↑Capillary permeability → Generalized edema, intravascular volume depletion. Clues: Fever, hypotension, inflammation.
• Lymphatic Obstruction: Impaired interstitial fluid drainage → Lymphedema. Clues: Non-pitting edema, history of surgery/radiation.

⭐ In nephrotic syndrome, massive proteinuria (typically >3.5 g/day) leads to decreased plasma oncotic pressure, a primary driver of generalized edema (anasarca).

Regulation of Balance - Hormonal Harmony

• ADH (Vasopressin): From posterior pituitary. Trigger: ↑plasma osmolality, ↓volume. Action: ↑$H_2O$ reabsorption (collecting ducts).
• Aldosterone (RAAS): From adrenal cortex. Trigger: ↓renal perfusion (Angiotensin II), ↑$K^+$. Action: ↑$Na^+$ & $H_2O$ reabsorption, ↑$K^+$ excretion (DCT/CD).
• Natriuretic Peptides (ANP/BNP): From heart. Trigger: Atrial/ventricular stretch. Action: Natriuresis, diuresis, vasodilation; oppose RAAS.

⭐ SIADH (Syndrome of Inappropriate ADH) leads to euvolemic hyponatremia due to excessive water retention and resultant sodium dilution.

High‑Yield Points - ⚡ Biggest Takeaways

• Osmolality differences drive water movement between ICF and ECF.
• Sodium (Na+) is the main determinant of ECF osmolality and volume.
• Water moves passively from lower to higher osmolality areas.
• Isotonic solutions (0.9% Saline) expand ECF volume without major ICF shifts.
• Hypotonic solutions (0.45% Saline) shift water into cells, causing ICF expansion.
• Hypertonic solutions (3% Saline) draw water out of cells, causing ICF contraction.
• Starling forces govern fluid exchange between plasma and interstitial fluid.