Osmosis

OSMOSIS

• Osmosis is passive transfer of solvent, especially water, across the membrane.
• Also, there is diffusion of solvent towards an area of higher solute concentration.
• Occurs if a semi-permeable (partially permeable) membrane is permeable to water but not to a particulate solute.
• Is second major passive transport mechanism after diffusion.

OSMOTIC PRESSURE:

• Minimum pressure required to prevent diffusion of solvent is “Osmotic pressure”.
• Depends on the number of osmotically active particles rather than particle type present in solution. Given by “Van’t, Hoff relationship“,π=nCRT, where
• n = number of dissociable particles per molecule
• C – total solute concentration
• R = gas constant
• T = temperature in degrees Kelvin.

Contributors:

• Plasma proteins have lower molar mass (weight) & high plasma molar concentration (number).
• Hence, contribute greatly to plasma osmotic pressure.
• Albumin with low molecular weight contributes more.

IMPORTANT & RELATED FACTORS:

MOLE:

• A mole (gram molecular weight) is molecular weight of a substance in grams.

OSMOLE:

• One mole of osmotically active particles is called “One osmole”.
• 1 osmotically active molecule (1 osmole) of a substance exerts osmotic pressure equal to 22.4 atmospheres.
• Major contributor to body fluid (ECF) osmolarity is Na+ followed by Cl-.

Types:

Ineffective osmoles –

• Molecules that are freely diffusible across the membrane do not generate osmotic driving force. 
• Eg: Glucose & urea.

Effective osmoles – 

• Molecules that are not freely diffusible (non-diffusible ions) across the membrane generate osmotic driving force. 
• Eg: Sodium & chloride ions.
• Causes “tonicity & osmosis of water”.

OSMOLARITY:

• The osmolar concentration of a solution in osmole/ liter is called “Osmolarity”.
• When expressed in osmole/Kg of solution is called “Osmolality”.
• Normal serum osmolality is about 290mOsm/Kg.
• ECF osmolarity is measured by osmometer.

Types of osmolar variations:

Iso-osmolar:

• Two solutions having identical osmolarity. 
• Exert same osmotic pressure & hence, iso-osmotic.

Hyper-osmolar/hyperosmotic:

• When one of the two solutions has greater osmolarity.

SOLVENT DRAG/BULK TRANSPORT:

• Refers to influence exerted by, allowing solvent through membrane on simultaneous solute movement through membrane.
• Important in renal & GI transport mechanisms.

OSMOSIS

• Osmosis is the passive transfer of solvent, especially water, across the membrane.
• Also, there is diffusion of solvent towards an area of higher solute concentration.

OSMOTIC PRESSURE:

• Van’t Hoff law is used for calculating osmotic pressure of a solution.

Contributors:

• Plasma proteins have lower molar mass (weight) & high plasma molar concentration (number).
• Hence, contribute greatly to plasma osmotic pressure.
• Albumin with low molecular weight contributes more.

IMPORTANT & RELATED FACTORS:

OSMOLE:

• 1 osmotically active molecule (1 osmole) of a substance exerts osmotic pressure equal to 22.4 atmospheres.
• Major contributor to body fluid (ECF) osmolarity is Na+.

Types:

• Ineffective osmoles – Eg: Glucose & urea.
• Effective osmoles – Eg: Sodium & chloride ions.
• Causes “tonicity & osmosis of water”.

“Osmolarity”:

• The osmolar concentration of a solution in osmole/ liter.
• Normal serum osmolality is about 290mOsm/Kg.
• ECF osmolarity is measured by osmometer.
• Mannitol infusion increases osmolarity.

SOLVENT DRAG/BULK TRANSPORT:

• Refers to influence exerted by, allowing solvent through membrane on simultaneous solute movement through membrane.