Organization of the Lymphatic System

Organization of the Lymphatic System

Lymphatic capillaries form tiny tubes called lymphaticpathways, which merge to form larger vessels, even-tually uniting with veins in the thorax. Microscopic lymphatic capillaries extend into interstitial spacesin complex networks (FIGURE 20-2). The walls of lym-phatic capillaries consist of a single layer of squamous­epithelium that allows tissue fluid to enter. The fluid inside these capillaries is called lymph. The normal components of lymph include water, plasma proteins, fats, and ions. Lymphatic capillaries are found in loose connective tissues between tissue cells and blood capillaries but are not found in bones, bone marrow, teeth, and all of the central nervous system. In the central­ nervous system, excess tissue fluid drains into the cerebrospinal fluid.

Proteins easily enter lymphatic capillaries but cannot enter blood capillaries. Inflammation of ­tissues causes lymphatic capillaries to develop open-ings through which larger particles can pass. These particles may include cancer cells, cell debris, and pathogens. The pathogens can then use the lym-phatics to travel elsewhere in the body. However, because lymph moves through the lymph nodes, the particles are usually removed and “evaluated” by the immune system cells.

Lacteals are special lymphatic capillaries locatedin the small intestine’s lining that absorb digested fats and carry them to the venous circulation. The name “lacteal” comes from the appearance of the lymph, which resembles milk. It is actually fatty lymph, known as chyle, which drains from the intestinal mucosa villi, which are finger-like in appearance.

Larger Lymphatic Vessels

Similar to veins but with thinner walls, lymphaticvessels have valves preventing backflow of lymph.Therefore, lymph moves through them in only one direction: toward the heart. Larger vessels lead to one of many bean-sized structures organized into clus-ters known as lymph nodes, and then continue on to form larger lymphatic trunks. Similar to veins, the collecting lymphatic vessels have three tunics butwith thinner walls. The vessels also have more internal valves and experience anastomoses more frequently. Basically, skin lymphatics are routed alongside super-ficial veins, but in the trunk and digestive viscera the deeper lymphatic vessels are found alongside the deep arteries. The exact locations of lymphatic vessels are more varied among different people than the distribu-tion of the veins.

The major lymphatic trunks of the body are the paired lumbar trunks, bronchomediastinal trunks, sub-clavian trunks, jugular trunks, and the single intestinal trunk. The lymph from lymphatic vessels drains as theyjoin one of two collecting ducts. FIGURE 20-3 depicts the right lymphatic duct and lymph drainage of the right upper limb and the right side of the head and thorax. The thoracic duct is larger and longer, receiv-ing lymph from the lower limbs, abdominal regions, left upper limb, and left side of the head, neck, and thorax. It empties into the left subclavian vein near the left jugular vein.

The thoracic duct arises anteriorly to the first two lumbar vertebrae as the cisterna chyli, an enlarged sac. It collects lymph from the lumbar trunks and the intestinal trunk. The superior portion of the thoracic duct receives lymph from the left side of the head, left upper limb, and left side of the thorax. On either side of the body, each terminal duct empties lymph into the veins where the internal jugular vein and subclavian vein join. The right lymphatic duct receives lymph from the right side of the head and neck, right upper limb, and right thorax. It empties into the right sub-clavian vein near the right jugular vein. Lymph then moves from the two collecting ducts into the venous system, becoming part of the plasma. This occurs just before the blood is returned to the right atrium.

Tissue Fluid and Lymph Formation

Lymph is basically the same as tissue fluid but is referred to as lymph once it has entered a lymphatic capillary. Tissue fluid is made up of water and dissolved sub-stances from the blood capillaries. It is very similar to blood plasma, containing gases, hormones, and nutri-ents. However, it lacks plasma proteins because their size does not permit them to leave the blood capillar-ies. Plasma colloid osmotic pressure helps to draw fluid back into the capillaries using the process of osmosis.

Lymph forms because filtration from blood plasma occurs at a higher rate than does reabsorption. The hydrostatic pressure of tissue fluid is increased, inducing tissue fluid movement into the lymphatic capillaries. Most of the small proteins the blood cap-illaries filtered earlier are returned to the bloodstream via the lymph. Lymph also carries foreign particles, including bacteria and viruses, to the lymph nodes.

Movement of Lymph

The movement of lymph is influenced by muscular activity because the lymphatic system has no organ that “pumps” lymph throughout its vessels. Lymph itself is under low hydrostatic pressure, and it moves similarly to how blood moves through the veins. Without contraction of skeletal muscles, smooth mus-cle contraction in the larger lymphatic trunks, and breathing-related pressure changes, lymph may not flow easily. Skeletal muscles, for example, compress lymphatic vessels to move the lymph inside, with valves preventing any backflow. Breathing creates a rel-atively low thoracic cavity pressure during inhalation, aiding lymph circulation. The diaphragm increases abdominal cavity pressure, squeezing lymph out of abdominal vessels and into thoracic vessels. Increased passive movement or physical activity causes lymph to flow more quickly. This balances the increased rate of fluid loss from the blood. Therefore, if a part of the body has a serious infection, immobilization results in decreased flow of inflammatory material out of it.

The continuous movement of lymph stabilizes fluid volume in the body’s interstitial spaces. When tissue fluid accumulates in the interstitial spaces, known as edema, it is because of an interference with lymph movement. Edema commonly occurs after sur-gery when lymphatic tissue is removed, such as when a breast tumor is removed. In this example, axillary lymph nodes may be removed as part of the surgery to prevent cancer cells from being transported via nearby lymphatic vessels. This can obstruct upper limb drain-ing, resulting in edema.

1. Identify the major components of the lymphatic system.

2. Differentiate between tissue fluid and lymph and include sources of both.

3. Describe from which parts of the body the left and right lymphatic ducts receive lymph.

4. Detail how lymph actually forms.

5. Explain the movement of lymph

Lymphoid Cells

The primary cells of the lymphatic system are lymphocytes . Lymphocytes are vital for the body’sability to resist or overcome diseases and infections and include T lymphocytes (T- cells) and B lymphocytes (B-cells). Plasma cells are a type of B lymphocytes that produce antibodies. Normal lymphocyte popu-lations are mostly maintained by the red bone mar-row. One type of lymphoid stem cell remains in the red bone marrow, whereas the other type migrates to the thymus gland. In the red bone marrow lymphoid stem cells divide, producing immature B cells and natural killer (NK) cells. The development of B cells involves close contact with large stromal cells inside the bone marrow.

Besides the lymphocytes, lymphoid cells include macrophages,dendritic cells,andreticular cells.Macrophages are vital for protection of the body and for immune response. They phagocytize for-eign substances and assist in the activation of T cells or T lymphocytes. Various macrophages are derived from monocytes. Spiked cells called dendritic cells capture antigens and transportthem to lymph nodes. Reticular cells are similar to fibroblasts and produce stroma, which is a reticu-lar fiber network that supports other lymphoid cell types. Macrophages are widely distributed through-out the connective tissues and lymphoid organs. They often present antigens to T cells for them to be activated. Some effector T cells release chemi-cals that activate macrophages, which are killer cells that both secrete bactericidal chemicals and actively phagocytize invaders.

Lymphoid Tissues and Lymphoid Organs

In the immune system, lymphoid tissue is very important for two major reasons. It contains lym-phocytes and provides a place for them to proliferate, and it gives lymphocytes and macrophages excellent areas to conduct their surveillance of various par-ticles. Lymphoid tissue is mostly made up of loose, reticular connective tissue. Except for the thymus,all lymphoid organs consist mostly of this tissue.

The macrophages are found on the reticular con-nective tissue fiber network. Many lymphocytes slip through the postcapillary venule walls of this net-work and occupy its spaces for a short period of time. Lymphocytes can reach sites of damage or infection quickly because of their regular cycling between lym-phoid tissues, circulatory vessels, and loose connective body tissues. You should remember that the primarylymphoid organs are only the thymus and bone mar-row. All other lymphoid organs are called ­secondarylymphoid organs.

The mucosa-associated lymphoid tissue pro-tects the epithelia of the respiratory, digestive, urinary, and reproductive systems. Aggregated lymphoid nod-ules or Peyer’s patches are clustered lymphoid nod-ules lying deep to the intestinal epithelial lining. The appendix and tonsils are also examples of mucosa-­ associated lymphoid tissue.

Lymph Nodes

Lymph nodes are actuallylymph glands. Totalingin the hundreds, they are found along the lym-phatic pathways and contain many lymphocytes and macrophages that fight invading microorganisms. Although they vary in size and shape, they are gen-erally bean-shaped and less than 2.5 cm in length (FIGURE 20-4). An indented region of each node, called the hilum, is where blood vessels and nerves are attached. In general, lymph nodes are hidden inside connective tissue structures called capsules. Large clusters are found near the surface of the body in the axillary, cervical, and inguinal regions. In these locations, lymphatic vessels merge and form the lymphatic trunks.

Each lymph node is enclosed and subdivided by a dense, fibrous capsule. Strands called trabeculae divide each node into compartments. A lymph node’s internal framework or stroma consists of reticular fibers, which support the lymphocytes that contin-ually change inside it. The cortex and medulla of a lymph node are distinct. Tightly packed follicles are contained in the superficial area of the cortex. Inside the medullary cords are areas where B cells germinate and divide. Follicles are almost totally surrounded by dendritic cells. The follicles touch the deeper cortex or paracortical area, which mostly contains tran-sitional T cells. These T cells circulate between the blood, lymph, and lymph nodes, continually mon-itoring particles. The medulla of each lymph node contains plasma cells and B cells organized into long medullary cords.

Both types of lymphocytes that exist are con-tained in the medullary cords, which are inward exten-sion from the cortical lymphoid tissue. The lymphsinuses are found throughout each lymph node.Many macrophages on the reticular fibers phago-cytize foreign matter in lymph that flows through the sinuses.

The functional units of a lymph node are the lymph nodules or follicles, which consist of B cellsand macrophages located in the node’s cortex. Lymph nodules occur either alone or in groups. The ton-sils are partially encapsulated lymph nodules, and groups of nodules called Peyer’s patches are found in the lining of the small intestine. Lymph sinuses are spaces inside a node that comprise complex channels through which lymph moves. There are more macro-phages in the lymph sinuses than in any other parts of a node.

Lymph nodes are grouped along larger lym-phatic vessels but do not exist in the central nervous system (FIGURE 20-5). Lymph nodes have two main functions: filter potentially harmful particles from the lymph before it is returned to the bloodstream and monitor body fluids. Immune surveillance occurs via the action of the lymphocytes and mac-rophages. Lymphocytes are produced in the lymph nodes and red bone marrow. They attack viruses, bacteria, and parasitic cells. Macrophages engulf and destroy cellular debris, damaged cells, and for-eign substances.

Lymph Node Circulation

A variety of afferent lymphatic vessels conducts lymph through the convex side of each lymph node. The lymph moves through a large subcapsularsinus into many smaller sinuses crossing the cortexand entering the medullary sinuses. It eventually leaves the lymph node at its hilum though efferentlymphatic vessels. There are more afferent vessels sup-plying the lymph node than efferent vessels draining it. Therefore, lymph becomes somewhat stagnant. During this slowed movement, macrophages and lymphocytes can “examine” the lymph more closely. The lymph passes through several lymph nodes before it is totally cleansed.

1. Describe the general functions of the lymph nodes.

2. Describe the general size of the lymph nodes and their primary locations in the body.

3. Explain lymphoid follicles and the lymphocytes that dominate in their germinal centers.

4. Why do lymph nodes have more afferent lymphatics than efferent lymphatics?

Thymus

The thymus is located in the thorax, anterior to the aorta and posterior to the upper sternum. It is soft and consists of two lobes enclosed in a connective tissue capsule ( FIGURE 20- 6). Although relatively large in infancy and early childhood, the thymus shrinks after puberty, becoming much smaller in adults. Its lymphatic tissue is replaced during the later years of life by adipose and connective tissues. The thymus is highly active during the first year of life. It continues to produce immunocompetent cells throughout life, but this production declines with aging.

The thymus is divided into lobules by inward-­ extending connective tissues or septa. Each lobule has a dense outer cortex and a central medulla that is paler in color. The lobules contain large amounts of lymphocytes, including primarily inactive thymo-cytes. Some thymocytes mature into T lymphocytes, which leave the thymus after three weeks and pro-vide immunity in the body. Thymosins are secreted by the thymus’s reticular epithelial cells and cause T lymphocytes to mature. In the medulla, these cells form thymic corpuscles, also called Hassall’s cor-puscles. In the cortex of the thymus, lymphocytes aredensely packed during their rapid division. Lesser numbers of macrophages are scattered throughout this area.

The thymus is different from other lymphoid organs in three major ways. First, it lacks B cells and therefore has no follicles. Second, the thymus does not directly fight antigens, unlike every other lymphoid organ. It is simply a place where T-lymphocyte precur-sors can mature and be isolated from foreign antigens so they are not prematurely activated. A blood–thymusbarrier stops bloodborne antigens from entering thethymus. Third, its stroma consists of epithelial cells and not reticular fibers. These cells create the chemical and physical environment needed for T-lymphocyte maturation.

Spleen

The spleen is located in the upper left abdominal cav-ity, inferior to the diaphragm and posterior and lateral to the stomach. It has the body’s greatest quantity of lymphatic tissue, and resembles a large, subdivided lymph node. The spleen is about as large as an adult’s fist and is attached to the stomach’s lateral border by a broad band of mesentery known as the gastrosplenicligament. The spleen is a soft organ that contains thelargest amount of lymphatic tissue and lymphoid nod-ules in an adult’s body. It differs from lymph nodes in that its venous sinuses are filled with blood, not lymph. The two types of tissues inside the splenic lob-ules (FIGURE 20-7) are white and red pulp.

■■White pulpis located throughout the spleenin small “islands,” made up of splenic nodules containing many proliferating lymphocytes. Immune function occurs in the white pulp, which is primarily made up of lymphocytes sus-pended on reticular fibers. Clusters of white pulp form around central arteries, which are the small splenic artery branches, making them appear as “islands” in the red pulp. The names “white pulp” and “red pulp” reflect their appearance in fresh spleen tissue, not how they stain to be microscop-ically examined. The trabecular arteries branch extensively, and white pulp surrounds their finer branches. Capillaries move blood into the red pulp.

■■ Red pulp fills the remainder of the lobules. Basi-cally, all splenic tissue that is not white pulp is considered to make up the red pulp. It contains many red blood cells and macrophages. In the red pulp, bloodborne pathogens and worn out red blood cells are destroyed. The red pulp con-sists of splenic cords, which are areas of reticular connective tissue. The splenic cords separate the blood-filled splenic sinusoids, which are also known as venous sinuses. The sinusoids empty into small veins that join the trabecular veins, con-tinuing toward the hilum. The blood capillaries of the red pulp are extremely permeable, and red blood cells easily squeeze through the capillary walls to enter the venous sinuses. Older red blood cells may be damaged during this process and so are engulfed by macrophages inside the splenic sinuses. Via the action of macrophages and lym-phocytes, the spleen filters blood similarly to the way that lymph nodes filter lymph. The large splenic artery and vein serve the spleen, entering and exiting its hilum on the concave anterior sur-face. Immune surveillance and response occur to a great degree in the spleen, but its cleansing of blood may be its most important function. Other functions of the spleen include storage of blood platelets and monocytes until they are required by the blood, storage of certain red blood cell breakdown products (such as iron) for reuse, and release of other breakdown products to be pro-cessed by the liver. The spleen may also be a site of erythrocyte production in the unborn fetus.

Tonsils

There are a variety of different tonsils, which are named according to their location. The tonsils create a ring of lymphoid tissue surrounding the entrance to the pharynx. They are seen as swellings of the mucosa. The two palatine tonsils are largest, the ones that usually get infected, and are found on either side of the posterior end of the oral cavity. The lumpy lymphoid follicles at the base of the tongue form the ­lingualtonsil. The pharyngeal tonsil is located in the pos-terior nasopharynx and is referred to as the adenoids when enlarged. The tubal tonsils are very small and surround the openings of the auditory tubes at the pharynx. Collectively, the tonsils collect and remove a variety of pathogens that enter the pharynx, in inhaled air, or in food.

The tonsils have predominant germinal centers in follicles, which are surrounded by scattered lympho-cytes. The epithelium over the tonsils continues deeply inside them to form tonsillar crypts. Therefore, the tonsils are not completely encapsulated. The tonsillar crypts collect bacteria and other particles. Most bac-teria are destroyed as they move through the mucosal epithelium into the lymphoid tissue of the tonsils. This procedure causes many immune cells to be produced that remember the various trapped pathogens.

Peyer’s Patches and Appendix

Peyer’s patches, also known as theaggregated lym-phoid nodules, are found in the walls of the distalsmall intestine (ileum). They are large, clustered lym-phoid follicles that resemble the tonsils. Along with the appendix, Peyer’s patches are located extremely well for the destruction of bacteria and also to generate “memory” lymphocytes. The appendix is tube-like, emerging from the first part of the large intestine. It contains many lymphoid follicles, although its actions are not fully understood.

1. If the thymus fails to produce thymic hormones, which population of lymphocytes will be affected?

2. Explain which organ contains the largest amount of lymphatic tissue.

3. Differentiate between white pulp and red pulp.

4. Explain the functions of the tonsils.

5. Define the terms Peyer’s patches and appendix.