Osmosis
Osmosis is the movement of water molecules across a semipermeable membrane from an area of lower solute concentration (higher water potential) to an area of higher solute concentration (lower water potential), until equilibrium is reached.
Key concepts:
- Semipermeable membrane - allows water to pass but restricts solutes
- Osmotic pressure - the pressure needed to stop osmotic flow; proportional to solute concentration
- Tonicity - describes how a solution affects cell volume:
- Hypotonic - less solute outside; water enters cell (cell swells/lyses)
- Isotonic - equal solute concentration; no net water movement
- Hypertonic - more solute outside; water leaves cell (cell shrinks/crenates)
- Osmolarity - total solute concentration (mOsm/L); drives osmotic gradients
Biological importance:
- Kidney tubules reabsorb water via osmosis
- Red blood cells maintain shape based on plasma tonicity
- Plant cells use osmotic pressure (turgor) for structural support
- Gut absorbs water following sodium/nutrient transport
Clinical relevance:
- IV fluid selection (normal saline, D5W, hypertonic saline) depends on osmotic effects
- Conditions like hyponatremia cause cerebral edema due to osmotic water shifts
- Dialysis exploits osmotic and diffusion gradients to remove waste
Is there a specific aspect of osmosis you'd like to go deeper on - physiology, clinical applications, plant biology, or something else?
Osmolarity
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Here is a thorough breakdown of osmolarity from authoritative medical sources:
Osmolarity
Definition
Osmolarity = the number of osmoles of solute per liter of solution (mOsm/L).
Osmolality = the number of osmoles of solute per kilogram of solvent (water) (mOsm/kg H₂O).
In clinical practice, the two values are nearly identical because 1 L of water ≈ 1 kg of water. The difference is less than 1% for dilute physiologic solutions, so the terms are often used interchangeably - but osmolality is what labs actually measure. - Morgan & Mikhail's Clinical Anesthesiology, p. 2135; Costanzo Physiology 7e, p. 19
How It Is Calculated
One osmole (Osm) = 1 mole of non-dissociable particles. For substances that ionize, each mole yields n osmoles, where n = number of ionic species:
- Urea (non-ionizing): 1 mol = 1 Osm
- NaCl (ionizes into Na⁺ + Cl⁻): 1 mol = ~1.86 Osm (not exactly 2, because ionic interactions reduce effective activity)
A difference of 1 mOsm/L between two solutions creates an osmotic pressure of 19.3 mmHg. - Morgan & Mikhail's Clinical Anesthesiology, p. 2135
Van't Hoff Equation for Osmotic Pressure
π = g × C × σ × R × T
| Symbol | Meaning |
|---|---|
| π | Osmotic pressure (atm or mmHg) |
| g | Particles per mole in solution (Osm/mol) |
| C | Concentration (mmol/L) |
| σ | Reflection coefficient (0-1) |
| R | Gas constant (0.082 L·atm/mol·K) |
| T | Absolute temperature (K) |
The reflection coefficient (σ) describes how well the membrane restricts a solute. σ = 1 means the solute is completely impermeable (maximally effective); σ = 0 means the solute crosses freely (no osmotic effect). - Costanzo Physiology 7e, p. 20
Calculated Plasma Osmolarity Formula
The standard clinical formula for calculated osmolarity:
Calculated Osmolarity = (2 × Na) + (Glucose/18) + (BUN/2.8) + (EtOH/3.7)
-
Na is in mmol/L; glucose, BUN, and EtOH are in mg/dL
-
Sodium (×2 to account for its anions) is the dominant contributor
-
Normal plasma osmolarity: ~285-295 mOsm/L
-
Rosen's Emergency Medicine, p. 2504
Osmolar Gap
Osmolar Gap = Measured osmolality - Calculated osmolarity
- Normal osmolar gap: ≤ 10 mOsm/kg
- An elevated gap (>10) signals the presence of unmeasured osmotically active solutes
Clinically important causes of elevated osmolar gap:
- Toxic alcohols: methanol, ethylene glycol, isopropanol
- Mannitol infusion
- Severe hyperlipidemia or hyperproteinemia (pseudohyponatremia)
Estimation tip: In toxic alcohol ingestion -
- Methanol concentration (mg/dL) ≈ osmolar gap × 3
- Ethylene glycol concentration (mg/dL) ≈ osmolar gap × 6
Note: A normal osmolar gap does NOT rule out toxic alcohol poisoning - the gap narrows as the alcohols get metabolized to their toxic acids. - Rosen's Emergency Medicine, p. 2504
Osmolarity vs. Tonicity
| Term | Meaning |
|---|---|
| Osmolarity | Total solute particles per liter (includes all solutes, even freely diffusible ones like urea) |
| Tonicity (effective osmolarity) | Only solutes that do NOT cross the membrane freely; determines actual cell volume change |
- Urea is an ineffective osmole - it crosses cell membranes freely, so it does not affect cell volume
- NaCl and glucose are effective osmoles - they stay outside cells and drive water shifts
An isotonic solution has no effect on cell volume; hypotonic solutions cause swelling; hypertonic solutions cause shrinkage. - Morgan & Mikhail's Clinical Anesthesiology, p. 2135
Regulation of Plasma Osmolarity
- Regulated primarily by ADH (vasopressin) and thirst
- Osmoreceptors in the hypothalamus detect changes as small as 1-2% in plasma osmolarity
- Rise in osmolarity → ADH release → water retention by kidneys → osmolarity falls back to normal
- The kidney can concentrate urine up to ~1200 mOsm/kg and dilute it as low as ~50 mOsm/kg
Quick Summary Table
| Feature | Osmolarity | Osmolality |
|---|---|---|
| Unit | mOsm/L solution | mOsm/kg water |
| Affected by temperature? | Yes (volume changes) | No |
| What labs measure | Calculated | Directly measured |
| Clinical difference | <1% in physiologic fluids | Reference standard |
Key numbers to remember:
- Normal plasma osmolality: 285-295 mOsm/kg
- Each 1 mOsm/L difference = 19.3 mmHg osmotic pressure
- Normal osmolar gap: ≤ 10 mOsm/kg
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