Oncotic Pressure

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oncotic pressure

[äŋ′käd·ik ′presh·ər]
Also known as colloidal osmotic pressure.
The osmotic pressure exerted by colloids in a solution.
The pressure exerted by plasma proteins.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Oncotic Pressure


the component of osmotic pressure that is contributed by substances of high molecular weight in a solution.

In human blood plasma, oncotic pressure ranges from 3 to 4 kilonewtons per square meter, or from 0.03 to 0.04 atmospheres; this constitutes only about 0.5 percent of the total osmotic pressure. Nevertheless, oncotic pressure plays a major role in the formation of some of the body’s fluids, including the intercellular fluid and capsular urine.

The walls of capillary blood vessels are readily permeable to water and substances of low molecular weight, but not to proteins. The filtration rate of fluid through the capillary wall is determined by the difference between the oncotic pressure, which is contributed by plasma proteins, and the blood’s hydrostatic pressure, which is a function of the heart. The saline solution at the arterial end of the capillary passes into the intercellular space together with nutrients. At the venous end of the capillary the process takes place in the opposite direction, since venous pressure is lower than oncotic pressure, and substances that are eliminated by the cells pass into the blood (see).

The oncotic pressure diminishes in diseases that involve a decrease in the concentration of blood proteins—especially albumins. This pressure decrease may be one of the reasons why fluid accumulates in the intercellular space and causes edema.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
* Colloids are suspensions of particles of various sizes that aim to maintain plasma volume by maintaining the colloid osmotic pressure in plasma, thus retaining the administered volume within the circulation.
Glomerular capillary colloid osmotic pressure ([[PI].sub.GC]), which is about 18 mmHg.
To understand the two main theories on the pathophysiologic mechanisms of NPE, one must first look to the pressure gradients that normally exist within the pulmonary circulation, the Starling forces.[11] The transcapillary fluid migration is governed by capillary hydrostatic pressure and interstitial tissue colloid osmotic pressure, which tend to favor movement of water out of the capillary, whereas plasma colloid osmotic pressure and interstitial tissue hydrostatic pressure tend to favor movement of water into the capillary.[11] Under normal conditions, a state of near equilibrium exists between these pressures.
Starling's forces The hydrostatic pressures that determine whether fluid moves out of the blood into the interstitium or the opposite direction: capillary pressure, interstitial fluid pressure, plasma colloid osmotic pressure and interstitial colloid osmotic pressure.
What triggers this hepatic response is not known, but several different authors have suggested that it might be hypoalbuminemia, with the consequent decrease in low plasma colloid osmotic pressure ([pi]) or low plasma viscosity ([eta]) [12-16].
The mechanisms of reducing the IOP by exercise were very complicated and believed to be associated with the lower concentration of norepinephrine, the rising of colloid osmotic pressure, the co-action of nitric oxide and endothelin after exercise, and also related to the gene polymorphism of [beta]2-adrenergic receptor.
Medical teaching on oedema usually focuses on Starling's equilibrium, ascribing much significance to decreased plasma colloid osmotic pressure in oedema formation.
The effects of varying volumes of crystalloid administered before Caesarean delivery on maternal hemodynamics and colloid osmotic pressure. Anaesth Analg 1996;83:299-303.