The energetics of lymph formation.

A very important unanswered problem is the source of the energy required for formation of lymph. The most widely held theory has been that fluid filters from the capillaries, flows through the tissues, and then enters the lymphatic as a result of a continuous positive pressure gradient from the blood capillary to the lymphatic capillary. However, this fails to account for the negative pressures that have been measured by implanted perforated capsules and by the wick technique, both of which suggest that the interstitial free fluid pressure is negative. Furthermore, because the fluid in the tissue gel is in equilibrium with the free fluid, it has a chemical potential equivalent to the negative pressure of the free fluid. At present, the only probable source of energy that could create the negative hydrostatic pressure in the free fluid and the equivalent negative chemical potential in the intragel fluid is a lymphatic suction pump. Unfortunately, the existence of this has not yet been proved, but mechanisms by which the terminal lymphatic system could act as a suction pump have been proposed.