Lymphatic anatomy and biomechanics

Abstract

Lymph formation is driven by hydraulic pressure gradients developing between the interstitial tissue and the lumen of initial lymphatics. While in vessels equipped with lymphatic smooth muscle cells these gradients are determined by well-synchronized spontaneous contractions of vessel segments, initial lymphatics devoid of smooth muscles rely on tissue motion to form lymph and propel it along the network. Lymphatics supplying highly moving tissues, such as skeletal muscle, diaphragm or thoracic tissues, undergo cyclic compression and expansion of their lumen imposed by local stresses arising in the tissue as a consequence of cardiac and respiratory activities. Active muscle contraction and not passive tissue displacement is required to support an efficient lymphatic drainage, as suggested by the fact that the respiratory activity promotes lymph formation during spontaneous, but not mechanical ventilation. The mechanical properties of the lymphatic wall and of the surrounding tissue also play an important role in lymphatic function. Modelling of stress distribution in the lymphatic wall suggests that compliant vessels behave as reservoirs accommodating absorbed interstitial fluid, while lymphatics with stiffer walls, taking advantage of a more efficient transmission of tissue stresses to the lymphatic lumen, propel fluid through the lumen of the lymphatic circuit.

Lymph flow pattern in pleural diaphragmatic lymphatics during intrinsic and extrinsic isotonic contraction

“…Given the strategic role played by pleural lymphatics in setting the correct pleural fluid volume and subatmospheric pressure required to maintain the normal lung chest wall coupling (29), much effort has been spent in the study of the inner regulatory mechanisms of lymph drainage and propulsion in this particular lymphatic network. Diaphragmatic lymph formation is sustained by net hydraulic pressure gradients (1,37) driving the entrance of fluid from the diaphragmatic interstitial space and from the pleural cavity into, respectively, the initial lymphatics and mesothelial stomata (28,31) in continuity with submesothelial lacunae (13,20). Subsequent lymph propulsion relies on the presence of hydraulic pressure gradients developing between two consecutive tracts of the lymphatic vessel, separated by unidirectional intraluminal valves.…”

A model to measure lymphatic drainage from the eye

“…Therefore, the act of injection may have biased the absorption somewhat in favour of the vasculature. Additionally, experimental conditions, such as general anesthesia (Quinn and Shannon, 1975), immobility and mechanical ventilation (Negrini and Moriondo, 2011) would have lead to reductions in lymph flow.…”

Spontaneous activity in peripheral diaphragmatic lymphatic loops

“…Transverse ducts depart from these vessels and run perpendicular to the diaphragmatic surface through the skeletal muscle fibers to reach larger collecting lymphatics, located in the center of the diaphragmatic thickness and supposed to propel the lymph away from the diaphragm (7). Throughout the body tissues, lymph formation and propulsion have been found to rely on two different mechanisms: one intrinsic, due to the rhythmic spontaneous contraction of the smooth muscle cells of the lymphatic vessel wall (12), and the other extrinsic, associated with tissue displacements and depending on the mechanical stresses arising in the tissue surrounding the lymphatics (15). Diaphragmatic lymphatic function has been extensively studied by means of fluorescence in vivo imaging and micropuncture technique in loops and linear vessels that are located in the muscular region just below the mesothelial layer.…”