Oncotic pressure

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.

It has an effect opposing both the hydrostatic blood 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.

However, this concept has been strongly criticised and attention has shifted to the impact of the intravascular glycocalyx layer as the major player.

The word itself is derived from onco- and -ic; 'onco-' meaning 'pertaining to mass or tumors' and '-ic', which forms an adjective.

In conditions where plasma proteins are reduced, e.g. from being lost in the urine (proteinuria), there will be a reduction in oncotic pressure and an increase in filtration across the capillary, resulting in excess fluid buildup in the tissues (edema).

The large majority of oncotic pressure in capillaries is generated by the presence of high quantities of albumin, a protein that constitutes approximately 80% of the total oncotic pressure exerted by blood plasma on interstitial fluid [citation needed].

It is necessary to understand the oncotic pressure as a balance; because the blood proteins reduce interior permeability, less plasma fluid can exit the vessel.

Within the capillary, reabsorption due to this venous pressure difference is estimated to be around 90% that of the filtered fluid, with the extra 10% being returned via lymphatics in order to maintain stable blood volume.

[8] In tissues, physiological disruption can arise with decreased oncotic pressure, which can be determined using blood tests for protein concentration.

[10] In the clinical setting, there are two types of fluids that are used for intravenous drips: crystalloids and colloids.

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.