Osmosis

Descriptions of Concentrations of Solute in Solvent in terms of total number of particles disregarding chemical activity of the individual particles
Semipermeable membrane	a membrane barrier to the passage of substances above a specific size, but which allows the movement through the membrane of substances below that size eg plasma membrane
osmosis	movement of a solvent [water] through a semipermeable [permeable only to the solvent] membrane from an area of low solute concentration to an area of high solute concentration
osmole	measure of the total number of particles eg 1 gram mole of nondiffusible & non ionizable substance = 1 osmole eg 0.5 gram mole of a substance that ionises into two ions (eg NaCl) = 1 osmole The osmotic pressure is determined by the number of particles instead of the mass of the solute
Osmolality	osmoles per Kg water
Osmolarity	osmoles per litre solution
osmotic pressure	pressure required to stop osmosis cmH20 or mmHg
tonicity	a relative term comparing osmotic pressure exerted by various solutions eg pure water is hypotonic relative to red cell [.307 osmoles]

Osmotic pressure & its units

• The pressure exerted on a semipermeable membrane separating a solution from a solvent, the membrane being impermeable to the solutes in the solution and permeable only to the solvent

• at body temp:

Osmotic pressure [mmHg] = 19.3 ´ Osmolarity (milliosmole/liter)

Osmolality

• osmoles per Kg water
• Technically more accurate as in very concentrated solutions, the osmotic pressure of a solution is considerably closer to osmolality than its osmolarity

Osmolarity

• osmoles per liter solution
• The total concentration of all solute particles in a solution
• Interchangeable with osmolality in weak solutions (as in body fluids)
• Usual term in physiology

Effect of molecular weight on osmosis

• Each nondiffusible molecule dissolved in water reduces the chemical potential of water by given amount. Consequently, the tendency of water in the solution to diffuse through a membrane is reduced in direct proportion to the concentration of nondiffusible molecules
• The osmotic pressure of the solution is proportional to the concentration of nondiffusible molecules in the solution
• This relationship holds true for all nondiffusible molecules almost regardless of their molecular weights
• Eg: one molecule of albumin (MW 70,000) has the same osmotic effect as a molecule of glucose (MW 180)
• The osmotic pressure is determined by the number of particles instead of the mass of the solute

Osmotic effect of ions

• Nondiffusible ions cause osmosis and osmotic pressure in exactly the same way as do nondiffusible molecules
• However, when a molecule dissociates into two or more ions, each of the ions exert osmotic pressures individually
• To determine the osmotic effect, all the nondiffusible ions must be added to all the nondiffusible molecules

Osmotic pressure of molecular solutions

Osmotic pressure (mmHg) = 19.3 ´ Osmolarity (milliosmole/liter)

1 milliosmole per liter concentration = 19.3 mmHg (38°C)

• The osmotic pressure is determined by the number of particles instead of the mass of the solute

eg: 0.9% NaCl

9 grams in 1 liter

1 mole Na = 23 g; 1 mole Cl = 35.5 g

23 + 35.5 = 58.5 g : 1 mole NaCl

9 g: X mole

X = 154 mmole NaCl

= 154 mmol Na + 154 mmol Cl

= 308 mOsm/liter

Osmotic pressure = 308 mOs/liter ´ 19.3

= 5944.4 mmHg

eg 1 mole dextrose in 1L —> 1 osmole/L

eg 1 mole NaCl in 1L —> 2 osmole/L

eg 1 mole CaCl2 in 1L —> 3 osmole/L

eg 1 osmole/L solution CaCl2 —> 1/3 mole or 111/3 = 37 gm

Exceptions to the osmotic pressure laws:

a) Strong solutions

• In very concentrated solutions, the osmotic pressure of a solution is considerably closer to osmolality [osmoles per Kg water] than its osmolarity [osmoles per liter water]

b) Electrolytes

• If particles interact with each other by either chemical or physical bonds, the osmotic effect becomes changed

§ All molecules and ions in solution exert either intermolecular attraction or repulsion resulting in a respective decrease or increase in the osmotic activity of the dissolved substance

• There is more intermolecular attraction than repulsion, so that the overall osmotic activity is about 93% of which one would calculate from the number of milliosmoles present
• Calculated mOsmole liter for plasma = 303
• Corrected osmolar activity = 283 mOsmole liter
• Eg: sodium & chloride in solution have a tremendous attractive power for each other so that neither of these ions are totally unrestrained: their chemical activities are normally 90% of what they would be without this attraction
• However, since osmotic effects in the body are usually determined by relative than absolute osmolar concentrations, the correction factor is often ignored