Mass and Fluid Transport in the Lymphatic System

Overview

The lymphatic system plays a vital role in fluid balance and homeostasis within the body. It is responsible for the transport of lymph from the interstitium to the venous return. The secondary lymphatics are composed of tubular structures segmented by check valves which are encapsulated by a bulbous sinus region. Intrinsic contractions powered by lymphatic muscle cells are influenced by the known vasodilator nitric oxide, NO. This outline details some of the key points in the development of a computational model to characterize the fluid dynamics and NO mass transport within the lymphatic system.

Characteristics:

  • Dyeing and scanning sample by confocal microscopy.
  • Segmentation and Meshing in Simpleware ScanIP +FE.
  • CFD simulation in Star-CCM+ from CD-adapco.
  • Analysis of stresses and concentrations.

Thanks to

Department of Biomedical Engineering, Texas A&M University 
J.T. Wilson • W. Wang • A.H. Hellerstedt • D.C. Zawieja • J.E. Moore Jr.

We gratefully acknowledge the Texas A&M University Supercomputing facility for providing computing resources useful in conducting the research reported in this outline.

Image Processing

Stacks of images were imported into Scan IP/FE (Simpleware, Exeter, UK) and refined using primarily recursive Gaussian, mean and median, and discrete Gaussian filters. Extensions were added to either end to allow for ease in the application of boundary conditions. The surface mesh was then imported into the commercial computational fluid dynamics solver Star-CCM+ for volume meshing and flow analysis.

Meshing

Following importation into the CFD solver, a surface wrapper was used to approximate the geometry and a volume mesh was created using the trimmer meshing module. This module produces a robust mesh that is primarily hexahedral with minimal cell skewness. The resulting mesh consisted of approximately 385,000 volumetric cells.

Simulations

A fully developed parabolic velocity profile with an average velocity of 2.0 mm s-1 was implemented at the inlet. A shear stress-dependent function was applied at the wall of the vessel as the flux boundary condition of NO. Simulations revealed areas of high concentration near upper surface of the valve leaflet corresponding to areas of essentially zero shear (Figure 2 Upper Panel). Additionally, high areas of high wall shear stress (WSS) were observed at the inner surface of the lymphatic valve leaflet.