Osmosis is the passive movement of water from an area high in water concentration, across a semi-permeable membrane, to an area low in water concentration. This movement achieves an equal amount of water on either side of the membrane. The blood contains a large number of plasma proteins with albumin constituting roughly 60% of it and thus there is less water content in the blood. This creates a concentration gradient between the blood and the fluid in the surrounding tissue. These proteins pull water into that compartment, as the force of osmosis tries to equalise the amount of water in blood and in the interstitial fluid and this pulling power is called oncotic pressure. Strictly speaking, all the dissolved compounds in blood exert an osmotic pressure. A part of this total osmotic pressure is due to the presence of large protein molecules and is called as the colloidal osmotic pressure, or oncotic pressure.
Normally, when oncotic pressure is measured, it is measured across a semi-permeable membrane, a membrane that is permeable to fluid and electrolytes but not to large protein molecules. In most capillaries, however, the wall (primarily endothelium) does have a finite permeability to proteins. The actual permeability to protein depends upon the type of capillary as well as the nature of the protein (size, shape, charge). There are other factors, which affect the mass-structure and the electrical charges of proteins that too exert a powerful influence upon the plasma oncotic pressure. Albumin generates about 70% of the oncotic pressure because of its concentration in the blood, its size and structure and this pressure is about 25-30 mmHg.