The Lymphatic System - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 243

Multiple lymphatic capillaries converge to form small lymphatic vessels which in turn
converge to form large lymphatic vessels. These vessels are comparable in wall structure
and size to venules and veins, respectively, and have a similar function as veins. Similar
to veins, lymphatic vessels contain valves that are spaced very close together to prevent
the backflow of lymph toward the interstitial space. At the valve location, the lymphatic
vessel wall bulges to accommodate the opening and closing of the lymphatic valves.
Without these bulges, the valves would not form a tight seal and there would be some
backflow of lymph. In vivo , lymphatic vessels appear to resemble a string of pearls because
the diameter changes abruptly and quite often at valve locations. Finally, after multiple
convergences, large lymphatic vessels join to form major lymph-collecting vessels that are
typically located within the abdomen or chest cavity.
Major lymph-collecting vessels are divided into two different types, the deep lymphatic
vessels and the superficial lymphatic vessels. The major difference between these two
types of lymph-collecting vessels is their location within the body. Deep lymphatic vessels
are located with the deep veins that collect interstitial fluid or blood from the skeletal mus-
cles of the limbs. Superficial lymphatic vessels are primarily located within the skin. Both
of these types of lymph-collecting vessels function to transport lymph to the collecting
lymphatic trunks which lead to the two main lymph-collecting vessels of the body. The
first major lymph-collecting vessel is the thoracic duct. The thoracic duct begins inferior to
the diaphragm and then runs upward through the chest cavity parallel to the aorta. The
major lymph vessels from the lower body and those from the left side of the upper body
transport lymph back to the cardiovascular system via the thoracic duct. The thoracic duct
terminates and empties its contents into the left subclavian vein, which is located near the
left clavicle. The second major lymph-collecting vessel is the right lymphatic duct. This
lymphatic vessel collects all of the lymph from the right side of the upper body. This duct
forms as a convergence of the major collecting ducts of the right arm and the right trunk.
This occurs near the right clavicle and then the vessel empties into the right subclavian
vein. For both major lymph-collecting vessels, the connection with the cardiovascular sys-
tem forms a closed loop circuit for the fluid that has exited the cardiovascular system, so
that the lymph that is collected near the blood capillaries is returned back into the cardio-
vascular system. This prevents the loss of fluid volume through capillary filtration and cel-
lular absorption.
The lymphatic system is also composed of lymphatic tissue and organs. The lymphatic
tissues are composed of many nodules that are associated with a large number of circulat-
ing lymphocytes. The two most important lymphatic tissues are the tonsils and the
mucosa-associated lymphoid tissue (MALT). The tonsils are large nodules located in the
pharynx. The MALT is all of the lymphoid tissues that are associated with the digestive
system. The majority of the MALT tissue is found within the lining of the intestines and
resembles the tonsils except that they are more spread out along the entire intestinal sys-
tem. The function of the lymphatic tissue is to provide a centralized location for the pro-
duction of mature lymphocytes, which can then function to protect the body against
inflammation.
Lymphatic organs are more specialized than the lymphatic tissue, and they consist of
the spleen, the thymus, and the lymph nodes. The spleen has a number of functions in the
body. First, it acts as a blood filter that removes damaged red blood cells, damaged white