The specific role of loops and/or linear segments in pleural diaphragmatic submesothelial lymphatics was investigated in 7 anesthetized, paralyzed and mechanically ventilated rats. Lymphatic loops lay peripherally above the diaphragmatic muscular plane while linear vessels run over both the muscular and the central tendineous regions. Lymph vessel diameter, measured by automatic software analysis, was significantly greater (p<0.01) in linear vessels (103.4 ± 8.5 (SE) µm; n=18) than in loops (54.6 ± 3.3 µm; n=21). Conversely, the geometric mean of intraluminal flow velocity (), obtained from the speed of distribution of a bolus of fluorescent dextrans injected into the vessel, was lower (p<0.01) in linear vessels (26.3 ±1.4 µm/sec) compared to loops (51.3 ±3.2 µm/sec). Lymph flow, calculated as the product of by vessel cross sectional area, was similar in linear vessels and in individual vessels of a loop, averaging 8.6 ± 1.6 nl/min. Flow was always directed from the diaphragm periphery towards the medial tendineous region in linear vessels, while it was more complex and evidently controlled by intraluminal unidirectional valves in loops. The results suggest that loops might be the preferential site of lymph formation, while linear vessels would be mainly involved in progression of newly formed lymph towards deeper collecting diaphragmatic ducts. Within the same hierarchic order of diaphragmatic lymphatic vessels, the spatial organization and geometrical arrangement of the submesothelial lacunae seem to be finalized at exploiting the alternate contraction /relaxation phases of diaphragmatic muscle fibers to optimize fluid removal from serosal cavities.