Kolloidontotisk

Kolloidontotisk (colloid oncotic pressure, COP) is a component of the forces that govern fluid exchange across capillary walls. It is the osmotic pressure generated by plasma proteins in the blood, primarily albumin. These proteins are largely confined to the intravascular space and thus pull water from the interstitial space back into the capillaries.

COP opposes hydrostatic pressure and helps maintain circulating blood volume. According to the Starling principle, fluid filtration at the arterial end of capillaries is driven by hydrostatic pressure, while reabsorption at the venous end is aided by COP. In practice, the exchange is influenced by factors such as the endothelial glycocalyx and local variations in pressure and permeability.

Determinants of COP include the concentration of plasma proteins, especially albumin. Conditions that reduce plasma albumin—such as liver disease, nephrotic syndrome, malnutrition, or acute inflammation—lower COP and can contribute to edema. Fluid therapies can affect COP: solutions containing proteins (for example, albumin) can increase COP more effectively than crystalloids, though their use depends on clinical context and potential risks.

Measurement and interpretation: COP is not routinely measured directly in clinical practice. It is typically inferred from serum albumin and total protein levels and interpreted alongside clinical assessment of volume status. Contemporary physiology also notes that capillary exchange is influenced by the glycocalyx and microvascular conditions, which can modify the effective impact of COP on fluid movement.