The second valve system in lymphatics.

The mechanism for interstitial fluid uptake into the lymphatics of the microcirculation remains speculative and uncertain. There exists a system of intralymphatic valves that prevent reflow along the length of the lymphatic channels, but these valves are insufficient to provide unidirectional flow at the level of the initial lymphatics. We propose that initial lymphatics have a two-valve system: a (primary) valve system at the level of the endothelium in addition to the classical (secondary) intralymphatic valves. The primary valves, in conjunction with the secondary valves, provide a mechanism that facilitates the unidirectional flow during periodic compression and expansion of initial lymphatics.