Sodium and Water Balance

Body Water & Compartments - Fluid Foundations

• Total Body Water (TBW):
  • Adult males: 60%; Females: 50-55%; Infants: 70-75%.
  • $TBW (L) \approx \text{Body Wt (kg)} \times 0.6$ (adult male).
• Compartments:
  • ICF: 2/3 TBW ($K^+$ major cation).
  • ECF: 1/3 TBW ($Na^+$ major cation).
    • ISF: 3/4 ECF.
    • Plasma: 1/4 ECF.

⭐ TBW measured with D2O, 3H2O; ECF with Inulin; Plasma with Evans Blue. ISF = ECF - Plasma; ICF = TBW - ECF.

Osmolality & Tonicity - Concentration Counts

• Osmolality: Measure of total solute concentration in a fluid.
  • Plasma Osmolality $≈ 2 \times [Na⁺] + [Glucose]/18 + [BUN]/2.8$
  • Normal range: 280‑295 mOsm/kg.
• Tonicity: Effective osmolality; determines water movement across cell membranes due to impermeable solutes.
  • Isotonic: No net water movement; cell volume unchanged.
  • Hypotonic: Water enters cell → cell swells.
  • Hypertonic: Water exits cell → cell shrinks.

⭐ Sodium and its accompanying anions (Cl⁻, HCO₃⁻) account for ~90% of ECF osmolality, making it the primary determinant of ECF volume and water distribution between ECF and ICF.

Sodium Homeostasis - Salty Sovereignty

• $Na^+$: Primary ECF cation; dictates ECF volume & osmolality. Daily intake: 3-5g.
• Regulation: Kidneys precisely manage $Na^+$ excretion.
  • RAAS: Renin → Ang I → Ang II (ACE) → Aldosterone. 📌 (Remember: R-A-A-A sequence)
    • Angiotensin II: Vasoconstriction, ↑Aldosterone, ↑ADH, ↑Thirst.
    • Aldosterone: ↑$Na^+$ reabsorption (late Distal Tubule & Collecting Duct), ↑$K^+$ excretion. Water passively follows $Na^+$.
  • Natriuretic Peptides (ANP, BNP): Counter RAAS; promote $Na^+$ excretion.
• ECV sensors: Carotid/aortic baroreceptors, juxtaglomerular apparatus (macula densa).

⭐ Aldosterone primarily acts on the principal cells of the late distal tubule and collecting duct, increasing ENaC and $Na^+$/$K^+$-ATPase activity.

Water Homeostasis - Aqua Guardians

• Key regulator: Antidiuretic Hormone (ADH)/Vasopressin.
• ADH release stimuli:
  • ↑Plasma osmolality (osmoreceptors; threshold ~280 mOsm/kg).
  • ↓Blood volume/pressure (baroreceptors).
• ADH action: ↑Water permeability in renal collecting ducts (AQP2 insertion) → ↑Water reabsorption → Concentrated urine.
• 📌 ADH: "Adds Da H₂O" (to body).

⭐ Osmoreceptors for ADH release are in the supraoptic & paraventricular nuclei of the hypothalamus.

Sodium Imbalances - Na‑sty Situations

Hyponatremia: Na+ <135. Severe: <125 mEq/L. Max correction 8-10 mEq/L/24h. Na+ Deficit: $TBW \times (Desired Na^+ - Measured Na^+)$.

⭐ Rapid correction of chronic hyponatremia (<125 mEq/L) can lead to osmotic demyelination syndrome (ODS).

Hyponatremia Types:

Hypernatremia (>145 mEq/L): Water loss/Na+ gain. 📌 6 D's (Diuresis, Dehydration, DI).

Water Imbalances - H2O Woes

• 📌 SIADH: Sodium Is Always Down Here.
• 📌 DI: Dry Inside (polyuria, polydipsia).

Diagnostic Approach to Polyuria:

High‑Yield Points - ⚡ Biggest Takeaways

• TBW is ~60% body weight; ICF 2/3, ECF 1/3. Na+ is main ECF cation.
• Plasma osmolality (280-295 mOsm/kg) reflects Na+; regulated by ADH (water balance).
• Aldosterone (RAAS) ↑Na+ reabsorption (water follows), ↑K+ secretion; regulates ECF volume.
• Hyponatremia: classify by volume. SIADH → euvolemic hyponatremia.
• Hypernatremia: usually free water deficit (impaired thirst, Diabetes Insipidus).
• Effective Circulating Volume (ECV), not total ECF, drives Na+ and volume regulation.