The tiny capillaries filter minerals, wastes and water but retain red cells, proteins and large molecules. This process is known as filtration. The proteins that are not filtered remain in the capillary and create an oncotic pressure due to osmosis. Filtration depends upon the surface area of the filter and the permeability of the membrane that surrounds the capillary. It also depends upon the systemic pressure and its counter, the pressure caused by osmosis.