I can't wrap my head around the idea of oncotic pressure and osmolarity, logically.

I imagine a blood vessel. It is filled with proteins, like a ton of proteins and solutes. So this means, according to oncotic pressure and osmolarity, that water will diffuse into this blood vessel.

I can't understand that. If I have a glass full of water, and I drop a bunch of rocks into the water (my proteins) the water will flow out of the glass. It will exert a force, a very basic force of buoyancy, that will lead water out of the glass.

How exactly is oncotic pressure generated?

  • 1
    $\begingroup$ blood vessel. It is FILLED with proteins - it's not. It filled with solution. And that solution differs by 0.5% of that osmotic pressure from cells solution. And that allows to move in- and out- cell, parts of solution from and in blood vessel. And direction of that movement will be directed by small changes(3-4kPa) in pressure. Which allows for blood vessels to do their job. So it's not like rocks and glass, it's like sponge, press harder get water, release and it will suck it. 3-4kPa is pretty close to difference between artery and veins pressures, and that helps direct where waste goes. $\endgroup$
    – MolbOrg
    Commented May 30, 2016 at 2:32

1 Answer 1


The molecular basis of osmotic pressure (oncotic pressure is a special type of osmotic pressure) is similar to filtration but on a very small scale.

Some membranes are partially permeable: they let small molecules pass but block larger ones. This asymmetry means that a flow of small molecules is created. Imagine things at the molecular level where a lot of individual motions are essentially random. The big molecules can't pass through the membrane but small molecules like water can pass through. If one side of the membrane consists of small molecules only and the other side has small and large molecules, then random motion will tend to lead to a net flow from of small molecules into the side containing large ones just because the concentration of small molecules is lower on the large molecule side. There are fewer opportunities for random small molecules to move from that side to the small molecule only side because there are fewer small molecules per unit volume on that side.

Obviously I'm simplifying a little, but this is the essential reason behind osmotic pressure.

  • $\begingroup$ I think the OP understands osmotic pressure but wants to know how macromolecules (which actually form a colloidal solution) exert similar kind of effects. Basically they are assuming that a macromolecule suspended in a solvent is like an inert rock in water. $\endgroup$
    Commented Jun 3, 2016 at 5:28
  • $\begingroup$ @WYSIWYG You might be right, but my explanation still holds. The explanation at the molecular scale isn't like dropping rocks into a glass: you have to think about the probabilities of water molecules passing through the membrane in different directions. $\endgroup$
    – matt_black
    Commented Jun 3, 2016 at 9:06

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