How can initial lymphatics expand and compress their lumens? Since these lymphatics have no smooth muscle media, their lumen compression and expansion depends on periodic motion of tissue structures positioned in proximity to the initial lymphatics. A variety of organ-dependent arrangements exist. For example, in skeletal muscle the initial lymphatics are located in the adventitia of the arterioles and in close apposition to each other. Expansion of the arterioles during vasodilation leads to compression of the adjacent lymphatic, whereas active contraction of the arterioles leads to expansion of the initial lymphatic vessel . Thus, both vasomotion and pulse pressure serve to promote lymph fluid . In addition, skeletal muscle contractions may also control the expansion and compression of the initial lymphatics. Thus, lymphatic pumping is a carefully controlled process closely linked to the physiological activity of the organ.
The pressure pulse leads to only low amplitudes of arte-riolar expansion and compression, so that the lymph flow generated in this fashion is relatively small. Lymph flow rates are greatly enhanced by either passive muscle stretching or active contractions. Cessation of pressure pulsations and vasomotion in a resting skeletal muscle leads to undetectable, near-zero lymph flow rates.
Many physical activities that generate cyclic compression and expansion of initial lymphatics also increase lymph transport. In addition to arterial pulse and vasomotion, rhythmic tissue deformations such as walking and running, heart muscle contractions, respiration, intestinal peristalsis, or skin massage enhance lymph flow. The faster the rate at which the initial lymphatics are pumped, the higher the lymph flow. The actual lymph flow rates achieved during such activities depend on the frequency of the cyclic tissue motions.
Lymph flow can also be enhanced by the elevation of capillary fluid pressure (e.g., elevation of venous pressure) and capillary filtration (e.g., elevation of endothelial permeability) as well as the elevation of interstitial fluid pressure, especially if the fluid pressure reaches values above those inside the initial lymphatics.
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