Oncotic pressure

In blood plasma, the dissolved compounds have an osmotic pressure. The difference between the osmotic pressure exerted by plasma proteins (colloidal osmotic pressure) in blood plasma and that exerted by tissue fluid proteins is called the oncotic pressure. Because large plasma proteins can't easily cross through the capillary walls, their effect on the osmotic pressure of the capillary interiors will, to some extent, balance out the tendency for fluid to leak out of the capillaries. In conditions where plasma proteins are reduced, e.g. from being lost in the urine (proteinuria) or from malnutrition, the result of the too low oncotic pressure can be edema – excess fluid buildup in the tissues.

It is represented by the symbol π

Related to hydrostatic pressure, starling equation.