Oncotic pressure


Oncotic pressure, or colloid osmotic-pressure, is a form of osmotic pressure induced by  proteins, notably albumin, in plasma that displaces water molecules.


Oncotic pressure creates a relative water molecule deficit with water molecules moving back into the circulatory system within the lower venous pressure end of capillaries. 


Oncotic pressure has the opposing effect of both hydrostatic blood pressure pushing water and small molecules out of the blood into the interstitial spaces within the arterial end of capillaries and interstitial colloidal osmotic pressure. 


Interacting factors determine the balance of total body extracellular water between the blood plasma and the larger extracellular water volume outside the blood stream.


Oncotic pressure  has a major effect on the pressure across the glomerular filter. 


Dissolved agents throughout the body have an osmotic pressure.


Large plasma proteins cannot easily cross through the capillary walls, and their effect on the osmotic pressure in capillaries balance out the tendency for fluid to leak out of the capillaries: the osmotic pressure tends to pull fluid into the capillaries. 


When plasma proteins are reduced,  there will be a reduction in oncotic pressure and an increase in filtration across the capillary, resulting in 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. 


Blood proteins cannot escape through capillary endothelium, rather oncotic pressure of capillary beds tends to draw water into the vessels. 


Blood proteins reduce permeability, and less plasma fluid can exit the vessel. 


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. 


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.


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