Osmosis and Osmotic Pressure

Osmosis Fundamentals - Water's Ways

• Movement of water (solvent) across a semipermeable membrane (SPM).
• Driven by water potential gradient: from area of ↑ water potential (↓ solute conc.) to ↓ water potential (↑ solute conc.).
• SPM: Selectively permeable; allows water, restricts most solutes.
• 📌 Water always follows solutes!
• Osmotic Pressure ($\\pi$): Hydrostatic pressure required to stop net water movement across SPM.
• Quantified by Van't Hoff's Law: $\\pi = nCRT$.

⭐ Osmosis is a colligative property; depends on the number of osmotically active particles, not their size or charge.

Osmotic Pressure - Push & Pull

• Osmotic Pressure (Π): "Pulling" pressure by solutes; pressure to prevent osmosis.
• Van't Hoff's Law: $Π = g \cdot C \cdot σ \cdot R \cdot T$
  • $g$: particle count (NaCl≈2, glucose=1)
  • $C$: molar concentration
  • $σ$: reflection coefficient
  • $R$: gas constant; $T$: absolute temp (K)
• Reflection Coefficient (σ): Solute permeability.
  • $σ = 1$: Impermeable (albumin); full osmotic effect.
  • $σ = 0$: Freely permeable (urea); no osmotic effect.
• Effective Osmotic Pressure: $Π_{eff} = σ \cdot Π$; dictates water flow.
• Oncotic Pressure: Colloid osmotic pressure by plasma proteins (e.g., albumin), crucial for capillary fluid exchange. Normal: ~25 mmHg.

⭐ Plasma osmolality: 280-295 mOsm/kg H₂O. Key contributors: Na⁺, glucose, urea. Calculated: $2 \cdot [Na⁺] + [Glucose]/18 + [BUN]/2.8$ (units: Na⁺ mEq/L, Glucose mg/dL, BUN mg/dL).

Osmolarity & Tonicity - Solution Showdown

• Osmolarity: Total solute concentration in solution.
  • Unit: mOsm/L.
  • Plasma: 280-295 mOsm/L.
  • Formula: Osmolarity = $g \times C$ ($g$=particles/mole, $C$=molar conc.).
• Osmolality: Solute concentration per kg solvent. Lab standard.
• Tonicity: Effective osmolarity; effect on cell volume. Depends on impermeable solutes.
  • Isotonic: No cell volume change. E.g., 0.9% NaCl, Ringer's Lactate.
  • Hypotonic: Cell swells (H₂O in). E.g., 0.45% NaCl.
  • Hypertonic: Cell shrinks (H₂O out). E.g., 3% NaCl, Mannitol.
• Key Distinction:
  • Osmolarity: All solutes (permeable/impermeable).
  • Tonicity: Only impermeable solutes; predicts H₂O shift.
  • 📌 E.g., Urea solution: isosmotic but hypotonic (urea enters cell).

⭐ D5W (5% Dextrose in water) is isotonic in vitro but physiologically hypotonic as dextrose is rapidly metabolized, leaving free water.

Clinical Correlations - Body Balance

Osmotic balance is crucial for cellular function and overall homeostasis. Disturbances lead to significant clinical issues.

• Fluid Imbalances:
  • Dehydration: Water loss > intake (e.g., vomiting, diarrhea). Leads to ↑ECF osmolality, cell shrinkage.
  • Water Intoxication: Excessive water intake or impaired excretion. Leads to ↓ECF osmolality, cell swelling (e.g., cerebral edema).
• Edema: Excess fluid in interstitial space.
  • Caused by: ↑Capillary hydrostatic pressure, ↓Plasma oncotic pressure, ↑Capillary permeability, Lymphatic obstruction.
• IV Fluid Therapy:
  • Isotonic (0.9% NaCl, Ringer's Lactate): No net water movement; for ECF volume expansion.
  • Hypotonic (0.45% NaCl): Water moves into cells; for cellular dehydration. Use with caution (risk of hemolysis, cerebral edema).
  • Hypertonic (3% NaCl, Mannitol): Water moves out of cells; for cerebral edema, severe hyponatremia. Use with caution (risk of osmotic demyelination).

⭐ In severe hyponatremia, rapid correction with hypertonic saline can lead to Osmotic Demyelination Syndrome (ODS). Correction rate should not exceed 8-12 mEq/L in 24h.

High‑Yield Points - ⚡ Biggest Takeaways

• Osmosis: Water movement via semipermeable membrane from low to high solute concentration.
• Osmotic pressure: Pressure to stop osmosis; depends on solute particle count.
• Van't Hoff's Law quantifies osmotic pressure; 'n' (dissociable particles) is key.
• Osmolality (mOsm/kg solvent) clinically preferred over osmolarity (mOsm/L solution).
• Tonicity (iso-, hypo-, hypertonic) dictates cell volume changes.
• Plasma osmolality: 275‑295 mOsm/kg; main solutes: Na+, glucose, BUN.
• Reflection coefficient (σ): 0 (freely permeable) to 1 (impermeable); impacts effective osmotic pressure.