Lymphatic pumping: mechanics, mechanisms and malfunction

Abstract

A combination of extrinsic (passive) and intrinsic (active) forces move lymph against a hydrostatic pressure gradient in most regions of the body. The effectiveness of the lymph pump system impacts not only interstitial fluid balance but other aspects of overall homeostasis. This review focuses on the mechanisms that regulate the intrinsic, active contractions of collecting lymphatic vessels in relation to their ability to actively transport lymph. Lymph propulsion requires not only robust contractions of lymphatic muscle cells, but contraction waves that are synchronized over the length of a lymphangion as well as properly functioning intraluminal valves. Normal lymphatic pump function is determined by the intrinsic properties of lymphatic muscle and the regulation of pumping by lymphatic preload, afterload, spontaneous contraction rate, contractility and neural influences. Lymphatic contractile dysfunction, barrier dysfunction and valve defects are common themes among pathologies that directly involve the lymphatic system, such as inherited and acquired forms of lymphoedema, and pathologies that indirectly involve the lymphatic system, such as inflammation, obesity and metabolic syndrome, and inflammatory bowel disease.

Keywords: lymphatic; lymphedema; muscle contraction.

Figure 1. Brightfield (A) and fluorescence (B) images of a popliteal afferent lymphangion from a Prox1GFP mouse after dissection, cannulation and partial cleaning

The vessel is pressurized to 3 cmH₂O from cannulation pipettes on either end (out of field of view). Calibration bars = 50 μm. Movies S1 and S2 in the online Supporting information show contraction sequences in each imaging mode.

Figure 2. Pump cycle of an isolated, cannulated (2‐valve) lymphangion from rat mesentery when P_out is elevated ramp‐wise while P_in is held constant

Normal direction of flow is left to right. Red and black diameter traces correspond to red and black tracking windows on each side of the output valve in the video image at the top. Blue and red densitometer traces correspond to blue and red densitometer windows positioned just upstream of the input and output valves, respectively. Valve position traces represent the binary state of each valve based on thresholding of the respective densitometer traces. Black pressure trace is the intraluminal pressure (between the valves) measured by a sharp servo‐nulling pipette advanced through (and sealed into) the wall. Modified from Davis et al. (2011).

Figure 3. Effect of elevating preload independently of afterload on the contractile function of an ex vivo mesenteric lymphangion from rat

A, P _in was elevated to various levels while P _out was held constant. Pipette resistances were purposely kept to relatively high values to limit the inhibitory effect of forward flow produced by P _in > P _out gradient. Inset shows diagram of pressure and diameter measurement sites. B, pressure–volume loop constructed from a portion of the data in A, showing the curvilinear P–V relationship for EDD (dashed line) and linear P–V relationship for ESD. C, time course of spontaneous contraction AMP and FREQ changes after a series of step elevations in P _in (P _out held constant). After each step, AMP falls but then recovers (or gets even larger) over the course of ∼1 min (arrow). Also, a burst of high FREQ contractions occurs, with FREQ subsequently slowing slightly. D, P–V plot of some of the data in C; blue traces represent data from 3 contraction cycles prior to the P _in steps (corresponding to time indicated by blue dot in C), black and gold traces represent single contraction cycles corresponding to the black and gold dots in C, immediately after the pressure step (black dots in C) or ∼1 min later (gold dots in C). The shift in the end‐systolic P–V relationship (ESPVR) with time after P _in elevation, with unchanged end‐diastolic (ED)PVR reflects an increase in contractility. Modified from Scallan et al. (2012 b).

Figure 4. Effect of elevating afterload in an ex vivo mesenteric lymphangion from rat

A, ramp‐wise elevation in P _out (with P _in held constant) leads to a progressive increase in the peak systolic pressure developed in the lumen (P _L, black trace). Note also a modest, progressive constriction on the input side of the valve during the pressure ramp. Opening of the output valve (top red trace) is indicative of ejection during systole, until P _out reaches ∼6.2 cmH₂O, at which point the pump limits (fails). B, response to a step increase in P _out. Contraction AMP declines initially but then partially recovers over the next ∼1 min. C, P–V plot of the data in B showing time‐dependent leftward shift in the curves after a P _out step (data in B represent the top set of curves). See diagram in Fig. 2 for explanation of pressure, diameter and valve position measurements. ESV, end‐systolic volume. Modified from Davis et al. (2012).

Figure 5. Ex vivo test for valve closure

A, valve closure test for a 1‐valve mouse popliteal lymphatic. An image of the mouse vessel with diameter tracking windows and position of the servo‐nulling pipette from which the black pressure trace in A was obtained is shown to the right. To measure the adverse pressure gradient required for valve closure, both pressures are set equal (valve is open), then P _out is elevated ramp‐wise (red pressure trace) until the valve snaps closed, as indicated by a rapid drop in the black trace; after a few more seconds, the P _out ramp is terminated. Dotted line shows alignment of changes in densitometer window positioned upstream from leaflets, servo‐null pressure and diameter on the input side of the valve at the point of closure. The ΔP for closure is the difference between P _out and P _in at the moment of valve closure. B, summary data for a single rat mesenteric lymphatic valve. ΔP for closure is <0.3 cmH₂O at low vessel diameters but rises to 2.2 cmH₂O when the vessel is near‐maximally distended. D represents diameter. Panel B modified from Davis et al. (2011).