Measure of pressure exerted by large dissolved molecules in biological fluids
Above, we see a representation of fluid flow in the presence of colloids, with the left side representing surrounding tissues and the right representing whole blood. The presence of colloids can increase the flow towards the high concentration of colloids by creating colloid osmotic pressure in an otherwise state of equilibrium.In the illustration above, we see how the osmotic pressure changes over the length of the capillary, with oncotic pressure remaining the same. Overall direction of fluid flow in relation to equal bidirectional flow is shown by the orange and black lines, respectively.
Oncotic pressure, or colloid osmotic-pressure, is a type of osmotic pressure induced by the plasma proteins, notably albumin,[1] in a blood vessel's plasma (or any other body fluid such as blood and lymph) that causes a pull on fluid back into the capillary. Participating colloids displace water molecules, thus creating a relative water molecule deficit with water molecules moving back into the circulatory system within the lower venous pressure end of capillaries.
It has an effect opposing both the hydrostaticblood pressure, which pushes water and small molecules out of the blood into the interstitial spaces at the arterial end of capillaries, and the interstitial colloidal osmotic pressure. These interacting factors determine the partitioning of extracellular water between the blood plasma and the extravascular space.
Oncotic pressure strongly affects the physiological function of the circulatory system. It is suspected to have a major effect on the pressure across the glomerular filter. However, this concept has been strongly criticised and attention has shifted to the impact of the intravascular glycocalyx layer as the major player.[2][3][4][5]
^Moman, Rajat N.; Gupta, Nishant; Varacallo, Matthew (2021), "Physiology, Albumin", StatPearls, Treasure Island (FL): StatPearls Publishing, PMID29083605, retrieved 2021-12-09