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:
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 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.
Starling forces determine fluid movement through capillary membrane:
Pc = Capillary pressure –> pushes fluid OUT of capillary
Pi = Interstitial fluid pressure –> pushes fluid INTO capillary
πc = Plasma colloid osmotic pressure –> pulls fluid INTO capillary
πi = Interstitial fluid colloid osmotic pressure –> pulls fluid OUT of capillary
Kf = Filtration constant (Capillary Permeability)
Net fluid flow = (Pnet)(Kf)
Edema: Excess fluid outflow into interstitium commonly caused by:
- INCREASE capillary Pressure (INCREASE Pc; Heart failure)
- INCREASE Capillary Permeability (INCREASE Kf; toxins, infection, burns, bradykinin)
- INCREASE Interstitial Fluid colloid osmotic pressure (INCREASE πi; Lymphatic blockage)
- DECREASE plasma protein (DECREASE πc; Nephrotic syndrome, liver failure)
References for Starling Forces: