Starlingligningen

Starlingligningen, commonly referred to as the Starling equation, is a principle in physiology that describes how fluid moves between capillaries and the surrounding interstitial space. It explains how hydrostatic forces within capillaries and oncotic forces from plasma proteins determine whether fluid filters into tissues or is drawn back into the bloodstream.

Mathematically, the net fluid flux across the capillary wall is given by Jv = Kf [(Pc − Pi) − σ(πc − πi)]. In this expression, Jv is the net filtration rate, Kf is the filtration coefficient reflecting capillary permeability and surface area, Pc is capillary hydrostatic pressure, Pi is interstitial hydrostatic pressure, πc is capillary oncotic pressure, πi is interstitial oncotic pressure, and σ is the reflection coefficient indicating how permeable the capillary wall is to proteins (ranging from 0 to 1). A positive Jv denotes filtration out of the capillary, while a negative Jv indicates reabsorption into the capillary.

The Starling equation provides a framework for understanding fluid balance in the body and is fundamental in contexts such as edema formation, fluid therapy, and cardiovascular or renal diseases. It highlights how changes in capillary pressure, plasma protein levels, or capillary permeability can shift fluid exchange toward filtration or reabsorption and thereby influence tissue hydration.

Historically, the concept was proposed by Ernest Starling in the late 19th century and has since been refined to reflect complexities such as the endothelial glycocalyx layer and dynamic capillary permeability. Despite refinements, the Starling equation remains a cornerstone of physiology for describing capillary fluid dynamics.