A major function of the lymphatic vasculature is maintaining fluid homeostasis by returning interstitial fluid from all of the body’s tissues to the venous system. Lymphatic pumping is achieved by a combination of external compression and intrinsic contractions coupled with one-way valves. Intrinsic contractions arise from specialised muscle lining the vessel walls. Lymphatic vessels must transport fluid against gravity and pressure differences without a central pump like the heart. This means that the muscle must fulfil the roles of both heart (phasic contractions, generate flow) and blood vessels (tonic contractions, regulate flow). Unfortunately, it is not possible to measure pressure, flow rate or diameter to any reasonable degree of precision in any lymphatic vessel in the human body. Using a combination of experiments in which vessels are either exposed or removed surgically, we can measure some combinations of these flow variables but not all simultaneously. We have also constructed mathematical models of lymphatic pumping, which have provided important insight.
Video of white blood cells being pumped through a lymphatic vessel. From Rahbar, et al., Microcirculation. 21(5): p. 359-67, 2014.