Multiscale Modelling of Lymphatic Drainage - Multiscale Computer Modeling in Biomechanics and Biomedical Engineering - page 149

2 Lymphatic Physiology
2.1 Primary Lymphatics
Primary lymphatics (initial lymphatics, prelymphatics or lymph capillaries) form
typically hexagonal or pentahexagonal capillary plexuses which are covered by a
layer of lymphatic endothelial cells. These cells are attached to the interstitium at
one end whilst the other end is free to move, but overlaps the adjacent lymphatic
endothelial cells (the end which is stuck to the intersititum). The suggestion is that
this construction creates a series of miniature fluid valves which ensure that the
fluid can move from the interstitium into the lymphatic lumen, but it cannot move
from the lymphatic lumen back to the interstitium [ 31 ]. The assumption about
there being a long thin channel between the overlapping endothelial cells is
somewhat debatable since in most of the histological, microscopical, and ana-
tomical representations of single primary lymphatic valves [ 23 ] there does not
appear to be any indication of this. However, clearly there can be exceptions and
possibly more thorough anatomical and experimental investigations into this issue
are needed. Irrespective of the details, the primary lymphatic vessels act to drain
the interstitial fluid and supply it to the larger, secondary lymphatics. Unlike the
secondary lymphatics, the primary lymphatics have no pumping action and so this
drainage must be a purely passive process (Fig. 1 ).
2.2 Secondary Lymphatics
The primary lymphatics feed lymph fluid into the next level, that of the secondary
lymphatics (lymph vessels or collecting lymphatics). The secondary lymphatics
are vessels with walls comprising smooth muscle, collagen and elastin fibres, lined
by endothelial cells and bound to surrounding tissue with fibrous adventitia [ 1 ]
(see Fig. 2 ). At intervals, simple valves occur, and the periodic contraction of the
smooth muscle in the walls interacting with the valves produces a pumping action
which transports the lymph fluid, despite any adverse pressure gradients, through
the system. This pumping action is thought to be enhanced by the proximity of the
secondary lymphatics to muscles, arteries and veins, which lead to a natural
mechanical forcing on the walls [ 42 ]. The ultimate destination of the lymph fluid is
the venous system, and as under normal conditions the fluid pressure in the veins is
higher than the fluid pressure in most tissues it implies that the lymphatic system,
i.e., the secondary lymphatics, can generate a reasonably large pumping force.
However there are conditions in which the reverse is true, such as edema, or cases
where limbs have been raised or subjected to compression for medical purposes.
The basic unit of a lymphatic vessel between two valves is referred to as the
lymphangion. As the lymph vessel accumulates more and more lymph from the
capillaries and connected lymphangions it swells. At intervals along the secondary