| Description |
The transport of lymph through the lymphatic vasculature is the mechanism for returning excess interstitial fluid to the circulatory system, and it is essential for fluid homeostasis. Collecting lymphatic vessels comprise a significant portion of the lymphatic vasculature and are divided by valves into contractile segments known as lymphangions. Despite its importance, lymphatic transport in collecting vessels is not well understood. We developed a computational model to study lymph flow through chains of valved, contracting lymphangions. We use the Navier-Stokes equations to model the fluid flow and the immersed boundary method to handle the two-way, fluid-structure interaction in 2D, non-axisymmetric simulations. We use our model to evaluate the effects of chain length, contraction style, and adverse axial pressure difference (AAPD) on cycle-mean flow rates (CMFRs). In the model, longer lymphangion chains generally yield larger CMFRs, and they fail to generate positive CMFRs at higher AAPDs than shorter chains. Simultaneously contracting pumps generate the largest CMFRs at nearly every AAPD and for every chain length. Due to the contraction timing and valve dynamics, non-simultaneous pumps generate lower CMFRs than the simultaneous pumps; the discrepancy diminishes as the AAPD increases. Valves exhibit hysteretic opening and closing behaviors. We modified the model by thickening the vessel wall in order to simulate pumping at higher AAPDs. We use the thick-walled model in the base-AAPD regime for parameter studies involving the transmural pressure (TP), interstitium, vessel wall, and valve leaflets. The CMFRs generally decrease as the TP decreases and as the interstitium becomes more resistant to flow. The CMFRs increase slightly as the vessel wall becomes more rigid; the CMFR trends vary in the simultaneous and non-simultaneous pumps as the leaflets become more rigid. Our model provides insight into how contraction propagation, AAPDs, TPs, and interstitial and material properties affect flow rates and transport through a lymphangion chain. |
