Multiscale Modelling of Lymphatic Drainage - Multiscale Computer Modeling in Biomechanics and Biomedical Engineering - page 171

6 Conclusions and Challenges
The lymphatic system acts to drain and return interstitial fluids to the main cir-
culatory system, pumping it from the low-pressure interstitial environment and
returning it to the (usually higher pressure) venous system via the lymph ducts.
This is accomplished by means of a complex structure of primary and secondary
lymphatics and lymph nodes spanning three orders of magnitude in scale. In
addition to its role in fluid transport, it provides a significant part of the body's
defense against disease, particularly through the intermission of the lymph nodes.
Medically, issues arise when compromised through surgery or infection, and it also
plays a role in the spread of cancer through the body.
The range of scales involved indicates the necessity to invoke multiscale
modelling; simulating individual components to investigate their behaviour, but
also using this investigation to create simplified models which can act as building
blocks for the next level up. In the case of the primary lymphatics, this has taken
the form of multiple scale perturbation expansion and has had success in
explaining the nature of fluid flow coupling between the interstitium and lymphatic
capillaries.
In the case of the secondary lymphatics, two basic approaches have been fol-
lowed; lumped-parameter modelling using electrical circuit analogies, and direct
solution of the Navier-Stokes equations in 1D. Both have been highly successful,
generating results which have been broadly in agreement both with each other and
with experiment, and have been used to discover aspects of the system behaviour
which would have been difficult to identify otherwise, such as coordination issues
and the non-pumping behaviour for the case of edema. A more extensive multi-
scale approach is starting to evolve within this area with detailed modelling of
valves beginning to inform the development of the network model. In all cases, a
synergistic approach involving experiment and modelling has been successfully
followed, partly as a pragmatic response to the lack of background information
available about physical parameters of the system.
Further modelling of the primary lymphatic system within the context of
detailed molecular factors influencing its development is clearly still needed.
There are two preliminary models [ 10 , 39 ], but many more detailed modelling
studies performed as part of experimental investigations are still needed. In the
secondary lymphatic system, advanced microscopy techniques [ 1 , 60 ] should be
more widely used to probe the structure of the secondary lymphatic vessel walls.
Further CFD modelling can investigate the detailed flow through valves and
lymphangions, possibly employing the techniques of Image Based Meshing to link
with the microscopy [ 59 ]. Coordination and integration of the modelling with the
primary lymphatics is also important, with the primary lymphatics providing input
boundary conditions for the secondary lymphatic models. The final aim must be a
complete, detailed model of the whole lymph system which can be used to
investigate not just drainage issues but propagation of components such as
malignant cancer cells through the system.