System Connections
Sara Lenz
HCS/230
August 1, 2011
Michael Allar, Ph.D.

System Connections
The Lymphatic System has four major structures lymphocytes, lymphatic vessels, lymphoid organs, and lymphoid nodules (Martini & Bartholomew, 2010). The functions of the lymphatic system are to provide the body with an immune system to fight off infections, bacteria, and any foreign invader that enters the body and maintains the balance of fluids in the internal environment (Cayuga School District, 1997). The lymphatic system works in correlation with the skeletal and cardiovascular systems (Allar, 2011).
The lymphatic system regulates fluids in the body (National Institute of Health, 2010). The importance of fluid regulation in the body is important to help prevent edema, a serious disease caused by an over-accumulation of bodily fluids that can lead to tissue destruction or death. This problem is avoided by the presence of lymphatic vessels that act as "drains" to collect the excess fluid and return it to the venous blood just before it reaches the heart (Cayuga School District, 1997). The lymphatic system is a specialized component of the circulatory system. It consists of a moving fluid (lymph) derived from the blood and tissue fluid and a group of vessels (lymphatics) that returns lymph to the blood. Lymphatic vessels parallel veins (Mitchell High School, 2005).
The Lymphatic System includes lymph nodes located along the paths of collecting vessels, isolated nodules of lymphatic patches in the intestinal wall, and specialized lymphatic organs such as the tonsils, thymus, and spleen. The lymphatic system differs from the circulatory system in that the lymphatics do not form a closed ring or circuit. Instead, it begins blindly in the intercellular spaces of the soft tissues of the body (Cayuga School District, 1997).
The fluids produced by the lymphatic system are called lymph and interstitial fluid. Lymph is a clear, watery fluid found in the lymphatic vessels. Interstitial fluid is fluid that fills the spaces around cells. In some cases, it is part of the semi-fluid ground substance, and in others, it is bound water in a gelatinous ground substance (National Institute of Health, 2010).
Lymphatic vessels are responsible for carrying the lymph and interstitial fluids through the body. Lymphatic vessels originate as microscopic blind-end vessels called lymphatic capillaries. Lymphatic capillaries merge with other lymphatics to eventually form the main lymphatic trunks (Cayuga School District, 1997).
Lymphatic vessels resemble veins in structure with these exceptions: lymphatics have thinner walls, lymphatics contain more valves, and lymphatics contain lymph nodes located at certain intervals along their course. Lymphatics play a critical role in homeostasis (Cayuga School District, 1997). The high degree of capillary permeability permits large molecular weight substances which cannot be absorbed by the blood capillary to be removed from the interstitial spaces. Proteins that accumulate in the interstitial spaces can return to the blood only via lymphatics. If anything blocks lymphatic return, blood protein and blood osmotic pressure fall below normal and fluid imbalance and death will result (Mitchell High School, 2005). Lacteals (lymphatics in the villi of the small intestine) serve an important function in the absorption of fats and other nutrients. The lymph found in lacteals after digestion contains 1-2% fat and is called chyle (National Institute of Health, 2010).
Water and solutes continually filter out of capillary blood into interstitial fluid. To balance this outflow, fluid continually reenters blood from the interstitial fluid. Each day about 50% of the total blood proteins leak out of the capillaries into the tissue fluid and return to the blood via the lymphatic vessels (Mitchell High School, 2005).
From lymphatic capillaries, lymph flows through progressively larger lymphatic vessels to eventually reenter blood at the junction of the internal jugular and subclavian veins (Martini & Bartholomew, 2010).
Although there is no muscular pumping organ connected with the lymphatic vessels to force lymph onward as the heart forces blood, still lymph moves slowly and steadily along its vessels. Lymph flows through the thoracic duct and reenters the general circulation at the rate of 125 mL/hour. This action occurs despite the fact that most of the flow is against gravity. It moves through the system in the right direction because of the large number of valves that permit fluid flow only in one direction. The movement is due to breathing movements and skeletal muscle contractions (National Institute of Health, 2010).
Lymph nodes are oval-shaped of bean-shaped structures (National Institute of Health, 2010). Some are as small as a pinhead and others as large as a lima bean. Each lymph node is enclosed by a fibrous capsule (Martini & Bartholomew, 2010). Lymph moves into a node via several different lymphatic vessels and emerges by one or two efferent vessels. Think of a lymph node as a biological filter placed in the channel of several afferent lymph vessels. Once lymph enters the node, it "percolates" slowly through the spaces known as sinuses before draining into a single efferent draining vessel. One-way valves in both the afferent and efferent vessels keep lymph flowing in one direction (Cayuga School District, 1997). Fibrous septa or trabeculae extend from the covering capsule toward the center of the node. Cortical nodules found within the sinuses along the outer region of the node are separated from each other by these trabeculae. Each cortical nodule is composed of packed lymphocytes that surround a less dense area called a germinal center. When an infection is present, germinal centers form and the node begins to release lymphocytes (Martini & Bartholomew, 2010).
Lymphocytes begin their final stages of maturation within the germinal center of the nodule and then are pushed to the more densely packed outer layers as they mature to become antibody-producing plasma cells. The center, or medulla, of a lymph node is composed of sinuses and cords. Both the cortical and medullary sinuses are lined with specialized reticuloendothelial cells (fixed macrophages) capable of phagocytosis (Martini & Bartholomew, 2010).
Lymph nodes perform defense for the body (National Institute of Health, 2010). Their main functions are filtration and phagocytosis. The structure of the sinus channels within the lymph nodes slows the lymph flow through them. This gives the reticuloendothelial cells that line the channels time to remove microorganisms and other injurious particles (soot) from the lymph and phagocytose them (Cayuga School District, 1997).
Sometimes such large numbers of microorganisms enter the node that the phagocytes cannot destroy enough of them to prevent their injuring the node. Because cancer cells often break away from a malignant tumor and enter the lymphatics, they travel to the node, where they may set up new growths. The lymphatic tissue of lymph nodes serves as the sites of the final stages of maturation for some types of lymphocytes and monocytes that have migrated from the bone marrow (Mitchell High School, 2005).
Masses of lymphoid tissue located in a protective ring under mucous membranes in the mouth and back of the throat are called tonsils. These structures help protect against bacteria that may invade tissues in the area around the openings between the oral and nasal cavities (Martini & Bartholomew, 2010).
The thymus is located in the mediastinum, extending up into the neck as far as the lower edge of the thyroid and inferiorly as far as the fourth intercostal cartilage (National Institute of Health, 2010). It is in an unpaired organ consisting of two pyramid-shaped lobes with delicate and finely lobulated surfaces. The pyramid-shaped lobes of the thymus are subdivided into small lobules by connective tissue septa that extend inward from a fibrous covering capsule. Each lobule is composed of a sense cellular cortex and an inner less dense medulla. Both cortex and medulla are composed of lymphocytes in an epithelial framework. Medullary tissue can be identified by the presence of rather large laminated spherical structures called thymic corpuscles or Hassall's corpuscles (Cayuga School District, 1997).
The thymus performs two important functions, it serves as a final site of lymphocyte development before birth and soon after birth the thymus begins secreting a group of hormones collectively called thymosin that enable lymphocytes to develop into mature T-cells for our immune system (National Institute of Health, 2010).
The spleen is located directly below the diaphragm, above the left kidney and descending colon, and behind the fundus of the stomach. It is roughly oval in shape. It is surrounded by a fibrous capsule with inward extensions that roughly divide the organ into compartments (National Institute of Health, 2010). Arteries leading into each compartment are surrounded by dense masses (nodules) of developing lymphocytes. Because of its whitish appearance, this tissue is called white pulp. Near the outer regions of each compartment is tissue called red pulp made up of fine reticular fibers submerged in blood that comes from nearby arteries. After passing through the reticular meshwork, blood collects in venous sinuses and then returns to the heart through veins (Cayuga School District, 1997).
The functions performed by the spleen are defense, hematopoiesis, blood cell and platelet destruction and a blood reservoir. As blood passes through the sinusoids, reticuloendothelial cell (macrophages) lining these venous spaces remove microorganisms from the blood and destroy them by phagocytosis. Monocytes and lymphocytes complete their development to become activated in the spleen (before birth, red blood cells are also formed in the spleen). Macrophages lining the spleen's sinusoids remove worn-out red blood cells and imperfect platelets from the blood and destroy them by phagocytosis (Martini & Bartholomew, 2010). It also breaks apart the hemoglobin molecules from the destroyed red blood cells and salvages their iron and globin content by returning them to the blood stream for storage in bone marrow and the liver. The pulp of the spleen and its venous sinuses store considerable blood which can be released during hemorrhage (Cayuga School District, 1997).
The lymphatic system is an integral part of our bodies. Without this system, we would not hope to survive, even in the womb. The systems work together to transport the lymph in order to carry it through the body and to be excreted or stored, whatever necessary.

References
Allar, M. Ph.D. (2011). Organ Systems Matrix. Retrieved from Michael Allar, Ph.D., HCS230 website.
Cayuga School District. (1997). Lymphatic System Outline. Retrieved from http://www.ccsf.edu/Resources/Faculty/jllim/documents/Lymphatic9.pdf Martini, F. H., & Bartholomew, E. F. (2010). Essentials of Anatomy and Physiology (5th ed.). San Francisco: Pearson/Benjamin Cummings.
Mitchell High School. (2005). Lymphatic System. Retrieved from http://mhs.mcsnc.org/UserFiles/Servers/Server_2082346/File/medscience/Medical%20Sciences%20I/I%20Lymphatic%20System%20%20M.pdf
National Institute of Health. (2010). Immune System Glossary. Retrieved from http://www.niaid.nih.gov/topics/immuneSystem/Pages/glossary.aspx