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The supply of oxygen and nutrients to tissues is performed by the blood system and involves a net leakage of fluid outward at the capillary level. One of the principal functions of the lymphatic system is to gather this fluid and return it to the blood system to maintain overall fluid balance. Fluid in the interstitial spaces is often at subatmospheric pressure, and the return points into the venous system are at pressures of approximately 20 cmH2O. This adverse pressure difference is overcome by the active pumping of collecting lymphatic vessels, which feature closely spaced one-way valves and contractile muscle cells in their walls. Passive vessel squeezing causes further pumping. The dynamics of lymphatic pumping have been investigated experimentally and mathematically, revealing complex behaviors that indicate that the system performance is robust against minor perturbations in pressure and flow. More serious disruptions can lead to incurable swelling of tissues called lymphedema.
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Supplemental Video 1: Lumped-parameter modeling simulation of pumping by a chain of lymphangions contracting synchronously in the presence of a positive transmural pressure (Δptm = pin − pe) of 3 cmH2O. Data are shown for the first lymphangion (indicated by the star in Figure 8). As shown in the graph of contraction activation M with respect to time t, pumping is initiated by lymphatic muscle activation (red), followed by relaxation (green) and inactivation (yellow). These colors are then used in the other panels to indicate corresponding times. Suction begins at t ≈ 2.5 s when the pressure p inside the lymphangion dips below the inlet pressure pin (solid line), after which the flow rate Q through the first valve peaks. The dashed line in the pressure versus time plot indicates the external pressure pe = 2 cmH2O. The loops in the graph of the transmural pressure Δptm versus the diameter D illustrate the time course alongside the passive behavior of the lymphangion (black), with the area of the loop defining the output work and change in diameter representative of the flow rate generated.
Supplemental Video 2: Lumped-parameter modeling simulation of pumping by a chain of lymphangions contracting synchronously in the presence of negative transmural pressure (Δptm = pin − pe) of −1 cmH2O. Data are shown for the first lymphangion (indicated by the star in Figure 8). Muscular contraction activation M follows the same time course and color coding as shown in Supplemental Video 1. There is no suction at t ≈ 2.5 s in this case because there is no positive transmural pressure to expand the vessel after muscular contraction. The pressure–diameter loop, which illustrates the time course alongside the passive behavior of the lymphangion (black), is so small as to be barely visible, indicating pumping failure.