oncotic pressure

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oncotic pressure (colloid osmotic pressure) The part of the osmotic pressure produced by colloids (i.e. proteins and other large molecules) present within the blood vascular system. The vessel walls are relatively impermeable to these molecules. Oncotic pressure largely offsets the hydrostatic pressure that tends to drive water from blood vessels into the extracellular spaces and tissues. Hence, if oncotic pressure falls, there is increased risk of fluid accumulation and swelling in tissues (oedema).