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oncotic pressure[äŋ′käd·ik ′presh·ər]
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.
IU. V. NATOCHIN