| dc.description.abstract | The lymphatic system returns interstitial fluid back to the blood circulation. They
have a network of vessels with numerous lymphangions, the segment of lymphatic
vessel between two unidirectional valves. The valves aid in transporting lymph against a
pressure gradient, in addition to the lymphangion pump which exhibit cyclical variations
in diameter. Like blood vessels, baseline lymphatic tone is regulated with changes in
transmural pressure; however, the transient response of lymphatic diastolic diameter
following changes in transmural pressure has not been studied. The lymphangion pump
is often described using cardiac analogies. However, since an active system empties into
another active system in a lymphatic vessel, the analogy cannot characterize the
principles governing optimal lymphatic vessel function. Furthermore, to optimize lymph
flow there is also a need to characterize the lymphatic network structure.
To characterize the transient diameter response of lymphatic segment, we used
post-nodal bovine mesenteric lymphangions in an isobaric preparation and measured the diameter response to a step change in pressure. An immediate active reduction in enddiastolic
diameter with each incremental increase in pressure was observed.
To identify the principles governing optimal lymphatic vessel function, we
applied the result obtained from optimizing the interaction of the heart-arterial system to
measured lymphangion pressure-volume relationships. We assumed that the slope of end
systolic pressure-volume relationship (Emax) is equal to the slope of end-diastolic
relationship (Emin) above a cutoff pressure and Emax>Emin below the cutoff pressure.
Unlike the heart, we found that stroke work is not optimized when Emax = Emin. However,
there is a region where lymph flow is insensitive to changes in transmural pressure.
To characterize the lymphatic network structure, we used an approximation of
time-varying elastance model. We found there is an optimal length for the lymphangion
when it produces maximal flow. To develop a fractal network model, we determined the
ratio of radius and ratio of length of lymphangion at a confluence. Using conservation of
mass and certain simplifying assumptions, we showed that the ratio of radius, as well as
ratio of length of upstream lymphangion, to the downstream lymphangion at confluences
is 1.26. | en |
