Sodium and Water Balance

Sodium and Water Balance - Water & Na+ Landscape

• Total Body Water (TBW): ~60% body weight (adult male), ~50% (female).
  • 📌 "60-40-20 rule": TBW ~60%, ICF ~40%, ECF ~20% of body weight.
  • ICF: 2/3 TBW; K+ major cation.
  • ECF: 1/3 TBW; Na+ major cation (Interstitial ~75%, Plasma ~25%).
• Plasma Osmolality: Normal 280-295 mOsm/kg H₂O.
  • Determines water distribution.
  • Calculated: $2 \times [\text{Na}^+] + [\text{Glucose}]/18 + [\text{BUN}]/2.8$.
• Sodium (Na+): Principal ECF cation; key for ECF volume & osmolality.
  • Normal serum: 135-145 mEq/L.

⭐ Effective osmolality (tonicity), mainly Na+ dependent, dictates water shifts between ICF/ECF.

Sodium and Water Balance - Key Controllers

• Core Regulators: ADH, RAAS (Aldosterone), ANP, Thirst.
• Antidiuretic Hormone (ADH):
  • Source: Posterior Pituitary.
  • Trigger: ↑Osmolality, ↓Volume.
  • Action: ↑$H_2O$ reabsorption (kidney collecting ducts).
• RAAS (Renin-Angiotensin-Aldosterone System):
  • Trigger: ↓Renal perfusion, ↓$Na^+$ (macula densa).
  • Angiotensin II: Vasoconstriction; ↑Aldosterone, ↑ADH, ↑Thirst.
  • Aldosterone: ↑$Na^+$ reabsorption, ↑$K^+$ secretion (distal nephron).
• Atrial Natriuretic Peptide (ANP):
  • Source: Atria (stretch).
  • Action: Natriuresis, diuresis; inhibits RAAS.
• Thirst:
  • Trigger: ↑Osmolality, ↓Volume, Angiotensin II.
  • Action: ↑Water intake.

⭐ Aldosterone primarily controls ECF volume by modulating sodium content; ADH primarily controls plasma osmolality by modulating water excretion.

Sodium and Water Balance - Dilution & Depletion Drama

• Fundamentals: $Na^+$ (ECF vol & osmolality). ADH/thirst (water). RAAS/ANP ($Na^+$).
• Hyponatremia (Serum $Na^+$ < 135 mEq/L): "Dilution Drama"
  • Symptoms: Neurological due to cerebral edema (nausea, headache, seizures).
  • Diagnostic Approach:

- ⚠️ Correction: Max **8-10** mEq/L/24h to prevent Osmotic Demyelination Syndrome (ODS).

• Hypernatremia (Serum $Na^+$ > 145 mEq/L): "Depletion Drama"
  • Cause: Net water loss (most common) or hypertonic $Na^+$ gain.
  • Symptoms: Thirst, altered mental status, weakness, seizures.
  • Key Causes: Diabetes Insipidus (central/nephrogenic), dehydration, excess saline.
  • Treat: Gradual correction of free water deficit.

⭐ SIADH: Euvolemic hyponatremia; urine inappropriately concentrated (Urine Osm > 100 mOsm/kg, Urine Na+ > 20 mEq/L) despite low serum osmolality.

Sodium and Water Balance - Dehydration & Salt Surges

• Hypernatremia: Serum Na⁺ > 145 mEq/L. Primarily a water deficit problem.
• Causes:
  • Net Water Loss:
    • Hypovolemic: GI (diarrhea), Renal (diuretics, osmotic diuresis), Skin (burns, sweat).
    • Euvolemic: Diabetes Insipidus (Central/Nephrogenic), Insensible losses (fever, tachypnea).
  • Net Sodium Gain:
    • Hypervolemic: Hypertonic saline/bicarb admin, primary hyperaldosteronism, Cushing's.
• Clinical Features: Thirst, CNS (lethargy, weakness, seizures, coma), signs of volume status.
• Management:
  • Correct underlying cause.
  • Calculate water deficit: $L = ext{TBW} \times [( ext{Serum Na}^+ / 140) - 1]$
    • TBW ≈ 0.5-0.6 × Body Weight (kg).
  • Slow correction: ↓ Na⁺ by ≤ 0.5 mEq/L/hr (max 8-10 mEq/L/day) to prevent cerebral edema.
  • Fluid choice guided by volume status (see flowchart).

⭐ High-Yield: Rapid correction of chronic hypernatremia (>48h) can cause iatrogenic cerebral edema due to osmotic shifts into brain cells.

High‑Yield Points - ⚡ Biggest Takeaways

• Total Body Water (TBW) is approximately 60% of body weight; Extracellular Fluid (ECF) is 1/3 of TBW, while Intracellular Fluid (ICF) is 2/3.
• Sodium (Na+) is the principal ECF cation and primary determinant of ECF osmolality; Potassium (K+) is the principal ICF cation.
• Plasma osmolality (normal: 275-295 mOsm/kg) is mainly determined by sodium, glucose, and urea.
• Antidiuretic Hormone (ADH) primarily regulates water reabsorption in the collecting ducts via aquaporin channels.
• Aldosterone controls sodium reabsorption (and potassium excretion) in the distal nephron.
• Classification of hyponatremia (hypovolemic, euvolemic, hypervolemic) is critical for guiding appropriate management.
• Hypernatremia invariably indicates a free water deficit, requiring water repletion.
• SIADH is a key cause of euvolemic hyponatremia characterized by inappropriately concentrated urine despite low plasma osmolality.