Osmosis and Tonicity

Osmosis and Tonicity

Diffusion vs. Osmosis

• Diffusion: Refers to the movement of solute particles.

• Osmosis: A passive process referring to the movement of solvent.

  • Specifically, osmosis is the movement of solvent from a solution with a lower solute concentration to a solution with a higher solute concentration.

  • In biological systems, the solvent is generally water.

Key Feature of Osmosis

• Osmosis occurs only through a selectively permeable membrane.

  • This membrane allows the solvent to cross but not the solute particles.

  • As a result, the solvent moves to the more concentrated solution, diluting it until both solutions have approximately the same concentration.

Tonicity

• Definition: Tonicity describes one solution's ability to cause osmosis relative to another solution.

• Application: Used to compare the solute concentration of two solutions, such as the cytosol (inside a cell) and the extracellular fluid (ECF - outside the cell).

• The ECF surrounding a cell can have three variations in tonicity:

Isotonic ECF (iso- = "same")

• Definition: An isotonic ECF has the same solute concentration as the cytosol.

• Osmotic Ability: Both solutions have an equal ability to cause osmosis.

• Water Movement: There is no net movement of water into or out of a cell in an isotonic solution.

• Cell Effect: The cell maintains its normal shape and volume.

• Examples:

  • Solutions given by intravenous (IV) administration are almost always isotonic.

  • 0.9\% NaCl solution.

  • 5\% dextrose in water.

Hypotonic ECF (hypo- = "below")

• Definition: A hypotonic ECF has a lower solute concentration than the cytosol.

• Osmotic Ability: The cytosol has more solute particles and thus a greater ability to cause osmosis than the ECF.

• Water Movement: Water will move into the cell by osmosis.

• Cell Effect: The cell may swell and burst (lyse) due to the influx of water.

• Examples:

  • 0.45\% NaCl.

  • Most sports drinks.

  • Pure water.

• Clinical Relevance: Hypotonic solutions are given as IV fluids when a patient has certain types of dehydration.

  • Caution: Pure water is never given as an IV fluid because it is far too hypotonic and can lead to patient death by causing cells (especially red blood cells) to lyse.

Hypertonic ECF (hyper- = "above")

• Definition: A hypertonic ECF has a higher solute concentration than the cytosol.

• Osmotic Ability: The ECF has a greater ability to cause osmosis compared to the cytosol.

• Water Movement: The ECF will pull water molecules out of the cytosol by osmosis.

• Cell Effect: The cell may shrivel or crenate (shrink) as it loses water to the ECF.

• Examples:

  • 3\% NaCl.

  • 10\% dextrose in water.

• Clinical Relevance: Such solutions are only ever given by IV under very specific and controlled medical conditions due to their drastic effect on cells.