3. Physiology
        3.4. General physiology
3.4.4. Osmotic pressure and oncotic pressure

Osmotic pressure and oncotic pressure

[Ref: KB2:p8,11-12]

Definitions

 

 

Osmotic pressure

... is the pressure necessary to prevent solvent migration (in osmosis)

For osmolality of 287mOsm/kg, the total plasma osmotic pressure is about 5540mmHg.

--> Can be calculated using van't Hoff equation

Van't Hoff equation

Osmotic pressure = n x (c/M) x RT

i.e.

Oncotic pressure

(aka colloid osmotic pressure)

.. is the component of osmotic pressure that is due to the colloids (i.e. often MW>30000).

Typical plasma oncotic pressure
= 25-28mmHg
= 0.5% of total osmotic pressure

Relative contribution

Of the main plasma proteins (albumin, globulin, fibrinogen)

Albumin

Albumin contribute most to oncotic pressure

Accounts for 65 to 75% of total value

Due to:

Fibrinogen

Fibrinogen contribute the least
* Due to lower concentration and high MW

Calculated and actual oncotic pressure

The actual oncotic pressure is higher than that calculated from van't Hoff equation (15mmHg) due to:

1. Donnan excess pressure

2. Excluded volume effect

Other notes

Oedema usually does not occur until albumin level is less than 20g/L

 

 

Keywords:  Diffusion ; Oncotic pressure ; Osmosis


Table of contents  | Bibliography  | Index