Oncotische

Oncotische, or oncotic, pressure is a form of osmotic pressure generated mainly by plasma proteins in the blood, especially albumin. This pressure tends to draw water from the interstitial space back into the vascular compartment, helping to maintain circulating blood volume and fluid balance.

In the capillary network, oncotic pressure opposes hydrostatic pressure. According to Starling forces, fluid filters out at the arterial end where hydrostatic pressure exceeds oncotic pressure, and is reabsorbed at the venous end where oncotic pressure dominates the local hydrostatic pressure. The balance between these forces determines net fluid exchange and the risk of edema.

Normal plasma oncotic pressure is roughly 25 mmHg, with interstitial oncotic pressure lower, around 5–10 mmHg. These values vary with protein concentration, particularly albumin, and are influenced by liver function, nutrition, and kidney loss. A decrease in oncotic pressure reduces the pull of water back into vessels, while relatively higher oncotic pressure can help retain intravascular volume.

Pathology related to oncotic pressure changes includes hypoalbuminemia, which lowers oncotic pressure and promotes edema and ascites, as seen in liver cirrhosis, nephrotic syndrome, malnutrition, or severe protein loss. Increased oncotic pressure is less common but can occur with high protein concentrations in certain conditions and treatments.

Clinical implications include fluid therapy considerations. Albumin or other plasma protein products can raise oncotic pressure to expand intravascular volume, whereas crystalloids mainly distribute into the interstitial space. The choice between crystalloid and colloid solutions depends on the clinical context and current evidence. The term oncotische (Dutch) refers to phenomena governed by colloid osmotic pressure.