Starling equation
The Starling principle holds that fluid movement across a semi-permeable blood vessel such as a capillary or small venule is determined by the hydrostatic pressures and colloid osmotic pressures (oncotic pressure) on either side of a semipermeable barrier that sieves the filtrate, retarding larger molecules such as proteins from leaving the blood stream. As all blood vessels allow a degree of protein leak, true equilibrium across the membrane cannot occur and there is a continuous flow of water with small solutes.