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Starling EquationFormulated in 1896 by the British physiologist Ernest Starling, the Starling equation illustrates the role of hydrostatic and oncotic forces in the movement of fluid across capillary membranes. According to Starling's equation, the movement of fluid depends on five variables: - Capillary hydrostatic pressure (Pc)
- Interstitial hydrostatic pressure (Pi)
- Capillary oncotic pressure (πc)
- Interstitial oncotic pressure (πi)
- Filtration coefficient (Kf)
All pressures are measured in millimeters of mercury (mm Hg), and the filtration coefficient is measured in milliliters per minute per millimeter of mercury. The Starling equation reads as follows: - Jv = Kf− Pi) − (πc − πi).
By convention, outward force is defined as positive, and inward force is defined as negative. The solution to the equation is known as the net filtration or net fluid movement (Jv). If positive, fluid will tend to leave the capillary. If negative, fluid will tend to enter the capillary. This equation has a number of important physiologic implications, especially when pathologic processes grossly alter one or more of the variables.
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