# What is the colloidal osmotic pressure?

The oncotic pressure or colloidal osmotic pressure is the osmotic pressure developed due to the presence of colloids in a solution. But since the colloids are not true solution, why should the colloids be termed as solutes soluble in the solvent and consequently capable of producing any osmotic pressure? Should they not behave like insoluble substances who do not affect the osmolarity of the solution? And theses colloidal osmotic pressure plays a very important role in trans-capillary transfer dynamics.

If the colloids do affect the osmotic properties, is the expression to measure the colloidal osmotic pressure same as that for true solutions(vant-hoff equation)?

• For a real world example of colloidal osmotic pressure, think of the albumin in human blood. This certainly exerts an osmotic pressure and among other things prevents most of the water in blood from entering the extracellular fluid compartment outside the circulation and causing oedema. – Tomcat Aug 10 '13 at 0:27

I think, first I should clarify what causes the osmotic pressure: Osmosis occurs when two solutions of different concentrations are separated by a membrane which will selectively allow some species, e.g. the solvent, through it but not others, e.g. the solute. So, there is a concentration gradient between the two solutions which would lead to a diffusion from the side with the higher to the side with the lower concentration. But since the solute cannot pass through the membrane the diffusive flow cannot even the concentration differences out and thus constantly presses against the membrane exerting a force on it which leads to a pressure. The pressure originating from this tendency for osmotic flow to occur is called the osmotic pressure $\Pi$ of the solution.