Oncotic pressure

In blood plasma, the dissolved compounds have an osmotic pressure. A small portion of the total osmotic pressure is due to the presence of large protein molecules; this is known as the colloidal osmotic pressure, or 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 serum proteins are reduced, eg from being lost in the urine (proteinuria), the result of the too low oncotic pressure can be edema - excess fluid buildup in the tissues.