Osmotic pressure and oncotic pressure
[Ref: KB2:p8,11-12]
Definitions
- Diffusion - the process by which a gas or a substance in solution expands to fill all of the available volume because of the motion of its particles.
- Osmosis - the diffusion of solvent molecules into a region in which there is a higher concentration of solute to which the membrane is impermeable.
- Nonionic diffusion - molecules of a undissociated substance diffuse to the other side of the membrane and then dissociates and unable to diffuse back, resulting in a net movement of the substance.
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.
- Hydrostatic pressure of 19.3192mmHg is equivalent to osmotic gradient of 1 mOsm/kg
* 0.082 x 310 x 760 = 19.3192
- Based on ideal solution
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:
- Higher concentration, lower MW
--> number of albumin molecule is much higher
- Negatively charged
--> Retention of sodium ions as per Gibbs-Donnan equilibrium
--> Increase in osmotic pressure
--> "Donnan excess pressure"
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
- Negatively charged protein retain sodium which increases osmotic pressure due to protein
2. Excluded volume effect
- Van't Hoff equation assumes ideal solution
- Large MW proteins occupies significant volume
Other notes
Oedema usually does not occur until albumin level is less than 20g/L