The Starling equation is an equation that illustrates the role of hydrostatic and oncotic forces (the so-called Starling forces) in the movement of fluid across capillary membranes.

Capillary fluid movement may occur as a result of three processes:

1. diffusion
2. filtration
3. pinocytosis

The starling equation incorporates the following variables:

• `J_v` - the output volume flux across capillary membranes reported in `"cm"^3/s`
• `P_c` - the capillary hydrostatic pressure
• `P_i` - the interstitial hydrostatic pressure
• `Pi_c` - the capillary oncotic pressure¹
• `Pi_i`- the interstitial oncotic pressure
• `K_f` - the filtration coefficient
• `sigma` - the reflection coefficient

¹  oncotic pressure is a form of osmotic pressure caused by proteins in blood plasma  that tends to draw water into the circulatory system.

Notes

Starling's equation only refers to fluid movement across the capillary membrane that occurs as a result of filtration. In the glomerular capillaries, there is approximately a net fluid filtration of 125 ml/min (about 180 litres/day). In the rest of the body's capillaries, there is a total net transcapillary fluid movement of 20 ml/min (about 28.8 litres/day) as a result of filtration. This is several orders of magnitude lower than the total diffusional water flux at the capillary membrane, as that is about 80,000 litres/day.

The filtration coefficient, `K_f`, id the product of the hydraulic conductivity, `L_P` and the capillary surface area, S.