The lymphatic system is an accessory route by which fluid can flow from the interstitial spaces into the blood. But how do the fluids manage to leave the blood vessels and enter the tissues? That is exactly what the English physiologist, Ernest Henry Starling (1866 to 1927), a founder of endocrinology and the man who coined the term hormone, wanted to know.

Starling tried to find the relationship between blood pressure and the behaviour of blood in the capillary system. According to theEncyclopaedia Britannica, Starling soon realized that the high pressure of the arterial system is enough to force fluids through the thin-walled capillaries into the tissues, but as the blood is divided through more and more capillaries its pressure falls. By the time it reaches the venous system the pressure of the fluid in the surrounding tissues is higher than that of blood in the venous capillaries, allowing much of the fluid lost from the arterial side to be regained.

In 1896 this scientist managed to demonstrate the Starling equilibrium: the balance between hydrostatic pressure causing fluids to flow out of the capillary membrane and osm otic pressure causing the fluids to be absorbed from the tissues into the capillary.

He showed that under normal conditions a state of near-equilibrium exists at the capillary membrane as the amount of fluid filtering outward from the arterial ends of the capillaries equals almost exactly the fluid returned to circulation by absorption. The slight disequilibrium that does occur accounts for the small amount of fluid that is eventually returned by way of the lymphatics. Under normal conditions the capillary pressure (30-40 mmHg) at the arterial end and 10-15 mmHg at the venous end of the capillary, tends to remain constant.

Starling’s equilibrium is largely dependent on four forces:

  • Capillary pressure or the pressure that forces fluid out of the capillaries
  • Plasma colloid osm otic pressure exerted by the tendency of proteins within the plasma to hold water
  • Interstitial fluid pressure that tends to force out fluid into the capillaries when positive and in when negative.
  • Interstitial fluid pressure exerted by the tendency of proteins in the interstitial fluid to hold water.