The lymphatic system has many roles in normal body function. It is a key component to body fluid and macromolecular homeostasis, lipid absorption, and lymphocyte circulation. To accomplish these tasks the lymphatic system must regulate lymph circulation. The lymphatics normally transport lymph and its components against pressure gradients. It does so through the involvement of active and passive lymphatic pumps and valved vessels. Many lymphatics have been shown to possess spontaneous cyclical vasomotor activity, which has been shown to be important to the generation of lymph flow. This active lymph pump can be analyzed in a fashion analogous to the cardiac cycle. Lymph pressure, rate of pressure change, diameter, rate of diameter change, contraction frequency, ejection fraction, stroke volume, flow, can be determined to analyze lymphatic contractile function. Many extrinsic factors are known to influence the active lymph pump, such as neurotransmitters, humoral factors, and inflammatory products. Influences intrinsic to the lymphatic vessel such as transmural pressure are also known to influence the active lymph pump. However the effect of another important intrinsic factor, flow is not clear. Lymph flow exhibits a complicated pattern during normal lymphatic function that shows great changes in magnitude and also transient changes in direction. If the lymphatic contractile activity is sensitive to shear, lymph flow is not only the main result of the lymph pumps but it may also be a feedback influence on these vessels. In spite of the potential importance of the effects of flow on lymphatic contractility, there are only a few reports of the influence of imposed flow on an active lymph pump. We have shown an inhibition of the amplitude and frequency of contractions induced by a controlled increase in imposed flow in isolated lymphatic vessels. Others have shown increases in the contraction frequency, reductions in lymphatic diameters and decreases in the amplitude of contractions in isolated rat iliac microlymphatics as a result of increases in flow. So, it is not clearly understood if and/or how changes in the magnitude of flow can modulate the contractility of different lymphatic vessels. Additionally the answers to several more questions concerning the influence of changes in flow conditions in lymphatics are also still unclear.
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