Oncotic

Oncotic refers to the colloid osmotic pressure exerted by proteins, especially in blood plasma. The term is most often used to describe plasma oncotic (colloid osmotic) pressure, the pulling force that plasma proteins exert to draw water from the interstitial space back into capillaries. The primary protein responsible is albumin, which provides the major share of this force, with smaller contributions from globulins and other plasma proteins.

Physiologically, oncotic pressure helps maintain intravascular volume and facilitates the reabsorption of water from the interstitial space into the vascular compartment. It acts in opposition to hydrostatic pressure that tends to push fluid out of capillaries. In contemporary physiology, the net movement of fluid across capillary walls is described by the Starling forces, where the oncotic gradient between plasma and interstitial fluid is a key determinant of fluid exchange.

Typical values place plasma oncotic pressure at about 25–30 mmHg, whereas the interstitial oncotic pressure is substantially lower due to limited protein presence in the interstitium. The exact contribution of oncotic pressure depends on plasma protein concentration, especially albumin levels, and the integrity of the capillary barrier.

Clinical significance arises when plasma protein levels fall, as in hypoalbuminemia from liver disease, nephrotic syndrome, malnutrition, or extensive burns. Reduced oncotic pressure can lead to edema, ascites, and reduced intravascular volume. Treatment considerations include addressing the underlying cause and, in selected cases, albumin administration, though routine use to treat edema is not universally warranted. Related terms include colloid osmotic pressure and the Starling forces framework.