Osmotic pressure and oncotic pressure

[Ref: KB2:p8,11-12]

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.

... 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

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

(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

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

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 contribute the least
* Due to lower concentration and high MW

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

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