Қан плазмасының онкотикалық қысымы және оның физиологиялық ролі

Measure of pressure exerted by large dissolved molecules in biological fluids

Oncotic pressure, or colloid osmotic pressure, is a type of osmotic pressure induced by the plasma proteins, notably albumin, 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 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. 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 been shifted to the impact of the intravascular glycocalyx layer as the major player.

The word 'oncotic' by definition is termed as 'pertaining to swelling', indicating the effect of oncotic imbalance on the swelling of tissues. The word itself is derived from onco and ic; 'onco ' meaning 'pertaining to mass or tumors' and ' ic', which forms an adjective.

Throughout the body, dissolved compounds have an osmotic pressure. Because large plasma proteins cannot 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 other words, the oncotic pressure tends to pull fluid into the capillaries. 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 The total oncotic pressure of an average capillary is about 28 mmHg with albumin contributing approximately 22 mmHg of this oncotic pressure, despite only representing 50% of all protein in blood plasma at 35 50 g/L. Because blood proteins cannot escape through capillary endothelium, oncotic pressure of capillary beds tends to draw water into the vessels. 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.

In tissues, physiological disruption can arise with decreased oncotic pressure, which can be determined using blood tests for protein concentration. Decreased colloidal osmotic pressure, most notably seen in hypoalbuminemia, can cause edema and decrease in blood volume as fluid is not reabsorbed into the bloodstream. Colloid pressure in these cases can be lost due to a number of different factors, but primarily decreased colloid production or increased loss of colloids through glomerular filtration. This low pressure often correlates with poor surgical outcomes. In the clinical setting, there are two types of fluids that are used for intravenous drips: crystalloids and colloids. Crystalloids are aqueous solutions of mineral salts or other water soluble molecules. Colloids contain larger insoluble molecules, such as gelatin. There is some debate concerning the advantages and disadvantages of using biological vs. synthetic colloid solutions. Oncotic pressure values are approximately 290 mOsm per kg of water, which slightly differs from the osmotic pressure of the blood that has values approximating 300 mOsm /L. These colloidal solutions are typically used to remedy low colloid concentration, such as in hypoalbuminemia, but is also suspected to assist in injuries that typically increase fluid loss, such as burns.