Revised 10/31/2007
The LYMPHATIC SYSTEM (FIG. 16.1) consists of a network of lymphatic vessels, lymph nodes, bone marrow, and lymphatic structures and organs, such as the spleen, thymus & tonsils.
LYMPHATIC VESSELS:
The lymphatic vessels are a system of vessels that contain a pale yellow fluid called LYMPH. Their function is to transport fluid that escapes from blood capillaries back to the bloodstream. This is important to maintain sufficient blood volume in the cardiovascular system.
Blood pressure forces some blood plasma out of the arterial ends of blood capillaries. See FIG. 16.2. Most of this plasma is reabsorbed by the blood capillaries, but what remains behind in the spaces between tissue cells becomes part of the INTERSTITIAL (TISSUE) FLUID. The nearby LYMPHATIC CAPILLARIES absorb excess interstitial fluid to prevent edema (tissue swelling). Once the pale yellow fluid is inside the lymphatic capillary, it is called LYMPH. About 3 liters of fluid per day are absorbed by lymphatic capillaries from tissues.
LYMPHATIC CAPILLARIES are found in most tissues that contain blood capillaries. See FIG. 16.2. The wall of the lymphatic capillary consists of ENDOTHELIUM. What type of tissue is endothelium?
Where else do you find endothelium?
Openings between the endothelial cells allow for absorption of excess interstitial fluid into the lymphatic capillary. During an infection, tissues become inflamed and swollen with interstitial fluid. The swelling pulls on ANCHORING FILAMENTS that are attached to the endothelial cells of the lymphatic capillary wall. This causes the endothelial openings to widen, so that more fluid can flow into the lymphatic capillary (along with bacteria and dead cell debris that may be in the infected tissue).
Lymph capillaries unite to form successively larger lymphatic vessels. See FIG. 16.1 & 16.3. LYMPHATIC VESSELS form a one-way lymph circulatory system in which lymph flows toward the heart. Lymphatic vessels closely resemble veins, but are thinner-walled and have more internal valves. LYMPH NODES (to be discussed later) are located at various intervals along the length of the lymphatic vessels. Lymphatic vessels of the integument are found in the subcutaneous tissue and they usually follow veins. Lymphatic vessels of the thoracic and abdominopelvic cavities are found near the aorta.
Lymph is eventually delivered from the lymphatic vessels into the largest lymphatic vessels, the LYMPHATIC TRUNKS. See FIG. 16.3. The lymphatic trunks empty into 2 large ducts in the thoracic region. The RIGHT LYMPHATIC DUCT drains lymph from the right arm and shoulder and the right side of the head, neck, chest, and thoracic cavity (see also the right-hand figure in FIG. 16.1). The THORACIC DUCT receives lymph from the rest of the body and is much larger than the right lymphatic duct. The thoracic duct arises anterior to the 2nd lumbar vertebra (L2) as an enlarged sac, the CISTERNA CHYLI. The cisterna chyli receives lymph from the lower limbs and the digestive organs. As the thoracic duct runs up along the anterior surface of the vertebral column, it receives lymphatic drainage from the left side of the thoracic cavity, left arm, and left side of the head and neck.
The THORACIC DUCT empties the lymph into the venous circulation at the junction of the LEFT INTERNAL JUGULAR VEIN and the LEFT SUBCLAVIAN VEIN. See Fig. 16.3. The RIGHT LYMPHATIC DUCT empties its lymph into the bloodstream at the junction of the RIGHT INTERNAL JUGULAR VEIN and the RIGHT SUBCLAVIAN VEIN. Remember: The ultimate goal of the lymphatic vessels is to return the fluid that has leaked from blood capillaries back to blood circulation.
LYMPHATIC TISSUE
Lymphatic tissue refers to the tissue found in the lymph nodes, spleen, thymus, tonsils, Peyer's patches, and lymphatic nodules. Lymphatic tissue consists of RETICULAR CONNECTIVE TISSUE (see TABLE 3.3, page 77) that contains large numbers of FIXED MACROPHAGES & LYMPHOCYTES.
Recall that MONOCYTES are produced in RED BONE MARROW. The monocytes move in the bloodstream to the lymphatic tissue, where they become FIXED MACROPHAGES. The LYMPHOCYTES are also produced in RED BONE MARROW. They travel in the bloodstream to the lymphatic tissue to proliferate (reproduce) and mature.
LYMPHATIC ORGANS of the body consist of LYMPHATIC TISSUE enclosed in a DENSE IRREGULAR CONNECTIVE TISSUE CAPSULE. The lymphatic organs are the LYMPH NODES, SPLEEN & THYMUS.
Other lymphatic tissue structures that are NOT surrounded by a dense irregular connective tissue capsule are the TONSILS, LYMPHATIC NODULES and PEYER'S PATCHES.
RED BONE MARROW (MYELOID TISSUE)
Since red bone marrow produces monocytes and lymphocytes, it is considered to be a component of the lymphatic system.
Red bone marrow consists of PLURIPOTENT HEMATOPOIETIC STEM CELLS & ADIPOCYTES embedded in a spongy fiber meshwork of RETICULAR CONNECTIVE TISSUE. The volume of red bone marrow in adults is 2-3 liters. See Fig. 13.3, page 410.
Where is red bone marrow found specifically?
LYMPH NODES
As lymph is transported through the lymphatic vessels, it is filtered through the lymph nodes. Particularly large clusters of lymph nodes occur near the body surface in the subcutaneous layer of the INGUINAL, AXILLARY and CERVICAL regions of the body. See FIGS. 16.1, 16.8, 16.9, 16.10. The inguinal, axillary and cervical regions are places where the lymphatic vessels converge to form large lymphatic trunks.
Deeper lymph nodes and lymphatic vessels in the thoracic & abdominopelvic cavities can be seen in FIGS. 16.11 & 16.12.
Note how the lymph nodes are found along the lymphatic vessels. The lymph nodes act as filters to cleanse lymph in the lymphatic vessels before the lymph is returned to the bloodstream. Within each lymph node is lymphatic tissue that contains fixed macrophages and lymphocytes (B and T cells).
What is the function of FIXED MACROPHAGES?
What is the function of B CELLS?
What is the function of KILLER (CYTOTOXIC) T CELLS?
What is the function of HELPER T CELLS?
Lymph nodes are 1-25 mm (.04-1 inch) in length. See FIG. 16.6. Each lymph node is surrounded by a DENSE IRREGULAR CONNECTIVE TISSUE CAPSULE. The capsule extends internally, forming internal walls called TRABECULAE. These trabeculae divide the inside of the node into compartments. The basic internal stroma (framework) of the lymph node consists of a soft network of RETICULAR CONNECTIVE TISSUE which is compartmentalized by the trabeculae. The internal stroma has 2 specialized regions: the CORTEX and MEDULLA. The outer CORTEX contains LYMPHATIC NODULES. Inside the nodules are GERMINAL CENTERS, which are the sites where B cells proliferate (divide) and mature into antibody-producing plasma cells. The cortex also contains macrophages and dendritic cells. Dendritic cells are phagocytic antigen-presenting white blood cells. They consume cells and present the cell's antigens on their cell surface to lymphocytes. The inner MEDULLA contains macrophages, B cell lymphocytes and T cell lymphocytes.
Circulation of lymph in the lymph node (FIG. 16.6): Lymph enters the convex side of a lymph node through a number of AFFERENT LYMPHATIC VESSELS. These vessels contain valves that open toward the node so that the lymph is directed inward. The lymph then moves through the inside of the lymph node through channels called SINUSES. The lymph meanders slowly through the CORTICAL SINUSES and then through the MEDULLARY SINUSES. The RETICULAR FIBERS in the sinuses act as filters to catch foreign particles so that the macrophages & lymphocytes can destroy them. The lymph finally exits from the lymph node at its HILUS (the indented region) via the EFFERENT LYMPHATIC VESSELS. Efferent lymphatic vessels contain valves that open away from the lymph node, so that lymph moves away from the lymph node. Note that there are several afferent lymphatic vessels, but only 1 or 2 efferent lymphatic vessels per lymph node. Why is this important?
SPLEEN: (See FIG. 1.9, page 16; & FIG. 16.7)
The spleen is the largest lymphatic organ of the body, measuring 12 cm (5 inches) in length. The spleen is located in the LEFT HYPOCHONDRIAC REGION of the abdominopelvic cavity, inferior to the diaphragm and posterior to the stomach.
The SPLENIC ARTERY (a branch of the CELIAC TRUNK) carries blood to the spleen. The celiac trunk is a branch of what artery?______________________________(Fill in the blank) The SPLENIC VEIN carries blood from the spleen to the HEPATIC PORTAL VEIN, which delivers blood to the _______________________________________. (Fill in the blank)
The spleen is surrounded by a DENSE IRREGULAR CONNECTIVE TISSUE CAPSULE. The spleen's capsule also contains scattered SMOOTH MUSCLE FIBERS.
The internal stroma (stroma = framework) of the spleen is called the SPLENIC PULP, which consists of RED PULP and WHITE PULP. RED PULP consists of VENOUS SINUSES that serve as sites where fixed macrophages remove old, worn-out formed elements and bacteria. Platelets are also stored in the red pulp of the spleen. The venous sinuses of the spleen also act as a BLOOD RESERVOIRS. During hemorrhage, the smooth muscle in the spleen capsule contracts, squeezing blood into the splenic vein (and, therefore, into circulation).
WHITE PULP consists of LYMPHATIC TISSUE and serves as a site of lymphocyte proliferation & maturation in response to infections.
THYMUS GLAND: FIGS. 16.1 & 16.5; FIG. 1.7, page 13
The thymus gland is located in the upper MEDIASTINUM, posterior to the sternum. The thymus is bilobed and resembles the leaf of the thyme plant. Each THYMIC LOBE is enclosed by a DENSE IRREGULAR CONNECTIVE TISSUE CAPSULE. The internal lobes contain LYMPHATIC TISSUE.
The thymus is the first organ to begin lymphocyte production in the fetus. The thymus functions primarily during childhood. The thymus "educates" the immature T cells about "self-antigen". 90-95% of T cells recognize "self-antigen" and undergo programmed cell death. The remaining 5-10% of T cells recognize only foreign antigen and these T cells then go on to proliferate and mature. Why is this is important in development of the immune system?
The thymus varies in size with age. See FIG. 16.5 (thymus of an adolescent). The thymus is prominent in newborns and continues to increase in size during childhood, when it is most active. It reaches its maximum size of about 40 grams during puberty. Its growth then stops and it starts to atrophy (degenerate) gradually. In a mature person, the atrophied gland continues to be a site of T cell proliferation. By old age it has been replaced entirely by fibrous connective tissue and fat, and is no longer functional.
MUCOSAL ASSOCIATED LYMPHATIC TISSUES (MALT):
MALT refers to lymphatic structures found in all mucous membranes, where bacteria can invade the body from the outside. Remember that mucous membranes line organs of the digestive, respiratory, urinary, and reproductive tracts. MALT structures include tonsils, lymphatic nodules & Peyer's Patches.
TONSILS:
The tonsils form a ring of lymphatic tissue around the entrance to the PHARYNX (throat), where they are embedded in the MUCOUS MEMBRANE. See FIG. 24.2, page 734 & FIG. 25.4, page 771.
Two PALATINE TONSILS are located behind the soft palate at the posterior end of the oral cavity. Two LINGUAL TONSILS lie at the base of the tongue. The single PHARYNGEAL TONSIL (ADENOID) is in the posterior wall of the pharynx, behind the nasal cavities. Tonsils consist of aggregations of large lymphatic nodules with no outer dense connective tissue capsule. The outer surface of the tonsils consists of deep crevaces in which bacteria and particulate matter become trapped.
The tonsils collect and remove many of the pathogens entering the throat region by inhalation or ingestion. Tonsils also serve as sites of lymphocyte proliferation and antibody production.
OTHER MALT STRUCTURES:
ISOLATED LYMPHATIC NODULES are located in the connective tissue layer (lamina propria) of the MUCOUS MEMBRANES of the digestive tract, respiratory passageways, urinary tract, and reproductive tract. See FIG. 25.2, page 767; FIG. 25.11, page 780; FIG. 25.17, page 790; and FIG. 25.20, page 796. Isolated lymphatic nodules protect the body from pathogens that may enter through body openings.
PEYER'S PATCHES are clusters of lymphatic nodules found in the mucous membrane of the lower small intestine. Peyer's patches prevent bacteria from penetrating the intestinal wall and entering the bloodstream.