Osmosis and tonicity

Water Movement and Osmosis

Learning Objectives

• Review body compartments relevant to water movement and osmosis.

• Learn the crucial value of body fluid osmolarity at 300 milliosmols per liter.

• Define osmosis and describe its role in driving water movement.

• Calculate osmolarity and tonicity through examples.

Body Compartments

• Body Compartment Types:

  • Extracellular Fluid (ECF): Fluid outside of cells.

  • Intracellular Fluid (ICF): Fluid inside the cells.

  • Interstitial Fluid (IF): Fluid between the cells, part of ECF.

  • Plasma: Liquid component of blood, also part of ECF.

• Understanding Water Movement: Water moves between compartments but must cross plasma membranes.

• Example of Water Movement:

  • Water from the external environment enters through ECF before reaching ICF.

  • Loss of water occurs during sweating, where sweat moves from ICF to ECF, then to the external environment.

Osmotic Equilibrium

• Definition: The concentration of particles (solutes) is equal in both ICF and ECF.

• Two mechanisms to achieve osmotic equilibrium:

  1. Movement of solutes.

  2. Movement of water.

• Plasma Membrane Permeability: Typically permeable to water but not to solutes (e.g., glucose).

• Water Movement Example: If solute concentration differs:

  • ICF has 6 milliosmols/liter.

  • ECF has 9 milliosmols/liter.

  • Water moves from ICF to ECF to achieve osmotic equilibrium.

Important Principles

• Water Follows Solutes: Water will move towards the compartment with higher solute concentration to maintain osmotic balance.

• Body’s natural behavior includes striving for osmotic equilibrium, maintaining osmolarity at approximately 300 milliosmols/liter.

Chemical Disequilibrium

• While osmotic equilibrium implies equal concentration of solutes, the specific types of solutes are different between ICF and ECF.

• ICF Composition: Predominantly potassium (K⁺) and proteins.

• ECF Composition: High sodium (Na⁺) and chloride (Cl⁻) concentrations.

• Key Concentration Differences:

  • High K⁺ inside cells, low Na⁺ inside cells.

  • High Na⁺ outside cells, low K⁺ outside cells.

Definition of Osmosis

• Definition: Passive movement of water across a semipermeable membrane in response to solute concentration gradient.

• Key Terms:

  • Concentration Gradient: Difference in concentration between two compartments.

  • Example: 300 milliosmols in ICF and 290 milliosmols in ECF results in a gradient of 10 milliosmols.

  • Passive Movement: Does not require energy (ATP).

Osmotic Pressure

• Osmotic Pressure: The pressure required to prevent water movement through a semipermeable membrane.

• Water movement direction can be predicted based on solute concentration. Higher osmotic pressure means more water movement.

• Example of Water Column:

  • Diagram representation shows water moving from regions of lower solute to higher solute concentration, increasing osmotic pressure and thus, water movement.

Osmolarity vs Molarity

• Osmolarity: Reflects concentration of all particles in a solution (milliomoles per liter).

• Molarity: Measures the total concentration of solute molecules in a solution (millimoles per liter).

• Key Differences:

  • Glucose remains as one particle in solution; hence osmolarity of a 10 mM glucose solution is 10 milliosmols/L.

  • Sodium chloride (NaCl) dissociates in water into two particles: osmolarity of 10 mM NaCl yields 20 milliosmols/L.

Calculating Osmolarity Examples

1. Sodium Chloride (NaCl): 100 mM --> 100 × 2 = 200 milliosmols/L.

2. Calcium Chloride (CaCl₂): 50 mM --> 50 × 3 = 150 milliosmols/L.

3. Glucose: 10 mM --> 10 × 1 = 10 milliosmols/L.

• Total Osmolarity of Mixed Solutions: Sum of all contributions from individual solutes:

  • 200 + 150 + 10 = 360 milliosmols/L.

• Comparison to Body Fluid: Solution has a higher osmolarity than body fluids; termed hyperosmotic.

Tonicity

• Definition of Tonicity: Refers to the effect of solute concentration on cell volume, specifically considers non-penetrating solutes.

• Non-Penetrating vs. Penetrating Solutes:

  • Non-Penetrating: Cannot cross membrane and affect water movement (e.g., Na⁺, Cl⁻).

  • Penetrating: Can freely cross the membrane and do not affect tonicity (e.g., urea, glucose).

• Calculating Tonicity:

  • Focus on non-penetrating solutes in a scenario to determine the effect on cell volume:

  • If ECF osmolarity < ICF osmolarity --> hypotonic (cell swells).

  • If ECF osmolarity > ICF osmolarity --> hypertonic (cell shrinks).

  • If ECF osmolarity = ICF osmolarity --> isotonic (no net water movement).

Example Problem on Tonicity Calculation

1. Calculate osmolarity of a solution:

   • Sodium Chloride (NaCl): 75 mM --> 75 × 2 = 150 mOsm/L.

   • Potassium Chloride (KCl): 30 mM --> 30 × 2 = 60 mOsm/L.

   • Glucose: 50 mM --> stays as one particle = 50 mOsm/L.

   • Urea: 20 mM --> stays as one particle = 20 mOsm/L.

   • Total Osmolarity: 150 + 60 + 50 + 20 = 280 mOsm/L.

   • This solution is hyposmotic to the cell (lower than 300 mOsm/L).

2. Calculate tonicity:

   • Focus only on non-penetrating solutes:

   • Tonicity (non-penetrating solutes): 150 + 60 + 50 = 260 mOsm/L (hypotonic).

3. Water Movement Direction: Water moves from ECF (tonicity = 260) to ICF (osmolarity = 300), causing cells to swell.

Summary of Key Concepts

• Osmotic Equilibrium: Achieving similar solute concentrations across compartments.

• Osmolarity: Total concentration of particles; normal value at 300 mOsm/L.

• Tonicity: Effect of solute concentration on cell volume using only non-penetrating solutes.

• Calculation Steps:

  1. Convert molarity to osmolarity.

  2. Calculate osmolarity by summing concentrated contributions of solutes.

  3. Determine tonicity focusing only on non-penetrating solutes.

  4. Assess water movement based on comparative tonicity and osmolarity.