Mechanics and fluid-dynamics of the initial lymphatic system.

The diaphragmatic lymphatic system drains fluid and solutes from the pleural and peritoneal cavities by exploiting pressure gradients (ΔPTM = Plymph-Pint ) between the vessel lumen and the interstitium; while lymph progression is sustained by pressure gradients (ΔPl = Plymph2–Plymph1) between adjacent lymphatic segments. The aim of the study is to examine the pattern of fluid fluxes in the lymphatics, determining the role of tissue and vessel wall in the transmission of tissue forces to the lymphatic lumen and clarifying whether the organization of the diaphragmatic network might be a tool to optimize lymphatic absorption and/or progression. Experiments were performed in anesthetized, paralysed and mechanically ventilated rats. Lymph hydraulic pressure and flux were recorded by the micropuncture technique combined with fluorescent dextrans injection to visualise lymph flow. Flow velocity and direction were calculated by measuring the fluorescence progression and vessel diameter on acquired videos. Mechanical compliance of the vessel wall was measured by sequential injections of saline solution into the vessel, while recording Plymph. The presence of muscular fibers around the lymphatic vessel wall was investigated by immunofluorescence assay. Two populations of vessels were identified with different mechanical properties: compliant vessels work as reservoirs of drained lymph, stiffer vessels propel fluid along the network. Flow direction and amplitude vary with time and, with respect to the Plymph pressure gradient, flow direction in some vessels reverts multiple times independently from both cardiac and ventilatory action.