Oncotic Pressure

The other force that contributes to fluid movement across the capillary wall is oncotic pressure. Oncotic pressure can be understood by recalling the nature of osmosis, which is the passive movement of water from an area high in water concentration, through a semi-permeable membrane, to an area low in water concentration. This movement achieves an equal amount of water in each area.

If we recall that blood contains a large number of plasma proteins, then we can conclude that these plasma proteins displace some of the water in the blood, and this means less water content in the blood. Less water in the blood compartment creates a concentration gradient between the blood and the fluid in the surrounding tissue. This means that in effect, these proteins pull water into that compartment, as the force of osmosis tries to equalize the amount of water in blood and in the interstitial fluid. This pulling power is called oncotic pressure.