The “useful” hindrance to flow: quantification of intraluminal valves effect on lymph flow driven by intrinsic mechanism in the diaphragmatic lymphatic network

The present study investigates the impact of intraluminal valves on lymph transport in rat diaphragmatic collecting vessels, with the aim of clarifying their role in intrinsically driven lymph flow. Using ex vivo fluorescent microsphere tracking and micropuncture techniques, lymph flow, pressure gradients, and hydraulic resistance were quantified in valved and nonvalved segments. Key findings revealed that valves significantly increased lymph velocity (approximately 1,617 µm/s) through their narrower functional section (∼14.4% of vessel cross section) compared with nonvalved segments (∼210 µm/s). The presence of valves required higher pressure gradients (4.15 ± 0.57 cmH2O vs. 2.16 ± 0.27 cmH2O in nonvalved segments) but markedly increased net lymph flow (68.0 ± 4.2% vs. 45.7 ± 3.7% of forward flow across valved and nonvalved tracts, respectively) by limiting reverse movement even if biased toward an open state. Despite increasing hydraulic resistance, lymph flow remained laminar, and valves optimized net lymph progression, particularly in larger vessels, where the ratio of net flow to forward flow was independent of vessel size. These results quantify and highlight the pivotal role of intraluminal valves in facilitating efficient, unidirectional net lymph transport, even under low-pressure, oscillatory flow conditions, by adapting to the unique hydraulic properties of the diaphragmatic lymphatic network. NEW & NOTEWORTHY This work quantifies the effect that intraluminal valves exert onto the intrinsic lymph flow in an ex vivo preparation of rat diaphragm. By means of particle tracking analysis and intraluminal hydraulic pressure measurements, the added pressure gradient due to the valve, its functional section and the positive effect on net lymph flow have been carefully measured in a quasiphysiological state, revealing that the open-biased state of the valve is the key to net lymph progression.