Oncotic pressure

Oncotic pressure, or colloid osmotic pressure, is a form of osmotic pressure exerted by proteins in blood plasma that usually tends to pull water into the circulatory system.

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) or from malnutrition, the result of low oncotic pressure can be excess fluid buildup in the tissues (edema).

Oncotic pressure is represented by the symbol π in the Starling equation and elsewhere.

In intravenous therapy

Clinically, 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; blood itself is a colloid.

Colloids preserve a high colloid osmotic pressure in the blood, while, on the other hand, this parameter is decreased by crystalloids due to hemodilution. ^[1] However, there is still controversy to the actual difference in efficacy by this difference.^[1] Another difference is that crystalloids generally are much cheaper than colloids.^[1]

References

External links

• Overview at physioweb.med.uvm.edu
• Overview at cvphysiology.com

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