LITMIR.BIZ

       THE FLOW OF LYMPH

      If the lymphatic system, as mentioned near the beginning of this chapter, does not have a muscular pumping organ (like the circulatory system does with the heart), how does the fluid get passed along through the various vessels and ducts? As an accessory route, this system does serve a unique transport function in that it returns tissue fluid, proteins, fats, and other substances to the general circulation. Yet there are differences from the true circulation of blood (as seen in the makeup of our cardiovascular system) to the flow of lymph. Unlike vessels in the blood vascular system, lymphatic vessels form only half a circuit, that is, they do not form a closed ring: there is not a continuous pathway with a “beginning” and an “end,” like the route of blood in its flow to and from the heart. Lymphatic vessels begin blindly in the intercellular spaces of the soft tissues of the body, collect the excess fluid there, finally draining it into the blood vascular venous system, and returning it to the heart.

      The two systems, though, closely parallel each other and work in conjunction with each other. In the skin, lymphatic vessels lie in the subcutaneous tissue (under the skin) and generally follow veins, while lymphatic vessels of the viscera (organs) generally follow arteries, forming plexuses (networks) around them. To understand more clearly how the lymph flows throughout the body, it is helpful to examine the composition and structure of lymphatic capillaries, which constitute the beginning of lymphatic vessels.

      All lymphatic vessels originate as lymphatic capillaries (also known as initial lymph vessels), tiny structures located in the spaces between the cells. Resembling a vast network of vessels, they form a fine mesh covering most of the body. Because their diameters are larger than blood capillaries, large substances that cannot be absorbed into a blood capillary (such as proteins) can be removed from the interstitial spaces and eventually returned to the blood.

      Lymphatic capillaries have a unique structure that permits interstitial fluid to flow into them but not out. The ends of the endothelial cells that make up the inner walls of the capillary overlap like roof tiles, called flap valves, permitting the influx of interstitial fluid. (See Fig. 4.) This process constitutes essentially the formation of lymph fluid.

      Just as blood capillaries converge to form venules, then veins, lymphatic capillaries also unite to form increasingly larger tubes (again like the branches and twigs of a tree). Though these larger lymphatic vessels resemble veins structurally, they have thinner walls and more valves, which allow the lymph to move in one direction only.

      Networks of lymphatic capillaries are distributed widely throughout the body. Yet some tissues lack lymphatic capillaries; these include avascular tissues (such as cartilage, the epidermis, the cornea of the eye), the central nervous system (the brain and spinal cord), splenic pulp, and bone marrow. In the small intestine, specialized lymphatic capillaries are called lacteals (lact, meaning “milky”). Located in the villi, they serve an important function in the absorption of fats and other nutrients, carrying dietary lipids (fats) into lymphatic vessels and ultimately into the blood. The presence of these lipids causes the lymph draining the small intestine to appear creamy white; such lymph is referred to as chyle (“juice”). In other areas, of course, lymph is a clear and pale fluid.

      Comparison of the lymphatic vessels to the blood circulation is helpful in understanding some of the similarities between the two systems. Another similarity has to do with transport. The same factors that assist venous flow also affect lymph flow. These include breathing movements and muscle and joint “pumps”; in addition to aiding the return of venous blood back to the heart, they maintain lymph flow as well. The so-called respiration pump involves, of course, breathing in and breathing out (inhalation and exhalation). Pressure changes occur in this process, and this helps to move the lymph along. For example, when we take a deep breath (diaphragmatic breathing), lymph flows upward from the abdominal region, where the pressure is higher, toward the thoracic region, where it is lower. When we breathe out (exhalation), the pressure is reversed, yet the one-way valves in the vessels prevent the backflow of lymph. Through the contraction of smooth muscles in the walls of the lymphatic vessel, lymph is moved from one segment of the vessel to the next. Research has shown that thoracic duct lymph is literally “pumped” into the venous system during inspiration. The rate of flow is proportional to the depth of inspiration.

      Also serving as lymph pumps are muscles and joints. When skeletal muscles, for example, contract, a “milking action” is created. This frequent intermittent pressure is put on the lymphatics to push the lymph forward. Most often this occurs during exercise and with general bodily movements. The contraction of muscles in both lymphatic vessels and veins force lymph eventually toward the subclavian veins (at the collar-bones). Compared to the blood vascular system, however, the pressure moving through the vessels is very low. Squeezing the fluid along this one-way system will ultimately drain it into the venous system. So the flow of lymph from tissue spaces to the large lymphatic ducts to the subclavian veins is maintained primarily by the contraction of joints and skeletal muscles and by breathing movements. During exercise, however, lymph flow may increase as much as ten- to fifteen-fold.

      Other pressure-generating factors that can compress the lymphatics also contribute to the effectiveness of the “lymphatic pump.” These include arterial pulsations (pulse waves), postural changes, and passive compression of the body’s soft tissues (manual lymph drainage therapy).

      Even though there is no central pump, lymph vessels themselves assist in transporting and pushing lymph. They do this through a self-activated pumping motion found in the lymphangions, the section in the lymphatic vessel between two valves. The interaction between these valves and the musculature of the vessel wall makes the contractions that propel the lymph forward. Because of this function, these little angions are called “lymph hearts,” which pulsate with an average frequency of ten per minute. In an X-ray, these vessels look like a string of pearls, with the string part representing the valves and the pearls the filled lymphangions. The ring-shaped muscles in the angions contain numerous nerve endings with connections to the autonomic nervous system; they are also influenced by the central nervous system. This is another way, along with the pumping motion, that the

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