Aim: 

In spite of its importance in the control of tissue fluid volume and composition, several functional aspects of the initial lymphatic system in thoracic tissues have not yet been described. The present study, aimed at providing a contribution to this issue examining the pattern of fluid kinetics in the submesothelial lymphatic lacunae.

Methods: 

Fluid kinetics in initial diaphragmatic lymphatics was studied in 7 anesthetized, paralyzed and mechanically ventilated rats. On opening the chest, two types of submesothelial lymphatic lacunae were distinguished under stereomicroscopic view: complex loops laying above the peripheral muscular plane of the diaphragm and linear vessels, visible both in the muscular and in the tendineous medial regions of the diaphragm.

Results: 

Lymph vessel diameter, measured by automatic software analysis of the vessel image, was significantly greater (p < 0.01) in linear vessels (103.4 8.5 (SE) mm; n = 18) than in loops (54.6 3.3 mm; n = 21). Conversely, the geometric mean of intraluminar flow velocity (v), obtained from the speed of distribution of a bolus of fluorescent dextrans injected in the vessel, was lower (p < 0.01) in linear vessels (26.3 1.4 mm/sec) than in loops (51.3 3.16 mm/sec). Lymph flow, calculated as the product of v by vessel cross sectional area, was similar in linear vessels and in individual vessels of a complex loop, averaging 8.6 1.6 nl/min. Flow direction was centripetal in linear vessels, more complex and evidently controlled by intraluminar unidirectional valves in loops.

Conclusion: 

The results suggest that loops might be the preferential site of lymph formation, while linear vessels would be mainly involved in centripetal propulsion of newly formed lymph towards deeper collecting diaphragmatic ducts.