Blood colloid osmotic, or oncotic, pressure is the pressure found within blood vessel plasma, according to Cardiovascular Physiology Concepts. This pressure is generated by proteins, most notably of which is a protein called albumin, which contributes about 70 percent of the total oncotic pressure found within the blood vessels.
Dissolved compounds such as albumin generate osmotic pressure throughout the body; it is the force that moves water in and out of cell membranes. Plasma proteins are large and cannot easily permeate capillary walls. Their size increases the interior pressure of capillaries and reduces the tendency of fluids to leak out. Because of this plasma-protein barrier, fluids are pulled into the capillaries. Therefore, oncotic pressure balances out the impact of capillary filtration pressure, which drives fluids out of the capillaries.
The interior pressure is more correctly referred to as colloid or oncotic pressure rather than osmotic pressure because it is created by substances dissolved in the blood plasma. In situations where blood plasma proteins are low, as in the case in malnutrition, oncotic pressure decreases, and capillary filtration increases. This imbalance leads to edema, or excessive fluid build up in the surrounding tissues. Understanding oncotic pressure and capillary filtration is especially important in the administration of intravenous drips in order to ensure that the fluid balance isn't unsettled.