The Lymphatic System - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 251

overcome the immune response and reach a very serious level. Also, with any edema, the
increased pressure can affect local tissue function and local blood flow.
Lymphedema may be inherited (primary lymphedema) or caused by lymphatic vessel
injury (secondary lymphedema). It is most frequently seen after lymph node dissection,
surgery, and/or radiation therapy, in which damage to the lymphatic system is caused
during the treatment. The exact cause of primary lymphedema is unknown, but it gener-
ally occurs due to missing lymph nodes or channels between the lymph nodes.
Lymphedema can manifest itself at birth, during puberty, or within adulthood. Secondary
lymphedema affects both men and women, but it occurs in different locations. In
women, lymphedema is most commonly found in the upper limbs. In men, it is most com-
monly found at the location of an injury or the legs. Regardless of the type or location of
the lymphedema, it develops in stages. The first stage, Stage 0, is termed the latent stage
and is classified by damage to the lymphatic vessels that has not become apparent yet.
Transport of interstitial fluid still occurs but it is typically shunted away from the dam-
aged vessels. Therefore, there is not a significant pooling of lymph. Stage 1 is termed the
spontaneously reversible stage and is characterized by pooling of lymph that reverses dur-
ing rest. In most cases, after depressing the affected area, an indentation will remain on
the surface for some time. Stage 2 is termed the spontaneously irreversible stage. This
stage is characterized by morphological and biochemical changes to the tissue due to the
presence of excess interstitial fluid. The affected region is hardened, and there is a general
increase in tissue size. The final stage, Stage 3, is termed the lymphostatic elephantiasis
stage. The swelling in this stage is irreversible, and the affected area is relatively large.
The tissue is very hard and does not depress when indented.

END OF CHAPTER SUMMARY
8.1

The lymphatic system is composed of lymphatic vessels, lymphoid tissues and organs,
lymph, and lymphocytes. Lymphatic vessels function to transport lymph away from micro-
vascular beds and into the cardiovascular system. The first vessel within the lymphatic sys-
tem is the lymphatic capillary, which is composed of loosely bound endothelial cells to
facilitate the movement of water into the lymphatic system. Lymphatic capillaries converge
into larger lymphatic vessels, which are similar in structure and function to venules.
Lymphatic vessels contain valves to prevent lymph backflow. The major function of lym-
phatic tissues/organs is to produce and control lymphocytes and to bring blood into a close
proximity with lymphocytes. Lymphocytes either directly destroy invading pathogens or
associate them with antibodies so that other cells can recognize and remove them.

8.2 Lymph is formed as a filtrate of interstitial fluid, except that there is typically a higher pro-

tein concentration in lymph than the interstitial fluid. The regulation of lymph formation is
therefore governed by the formation of interstitial fluid. Recall that the major players in
interstitial fluid formation are the capillary hydrostatic pressure, the interstitial space hydro-
static pressure, the plasma colloidal osmotic pressure, and the interstitial colloidal osmotic
pressure. There is an added component that lymph can be continually formed by cells within
the lymphatic system. Therefore, lymph formation can be described by

m 5 K p ðP B 2 P I 2 Π B 1 Π I Þ 1 m t