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In this lesson, we will cover the lymphatic system. The human body harbors just as many bacterial cells as human cells. Some of these bacteria are beneficial or even necessary to human health, and others have the capacity to cause disease. The lymphatic system includes organs, vessels, and tissues that consist of immune cells that help defend the body from agents of disease by recovering excess tissue fluid, inspecting it, cleaning pathogens out, and returning it to the bloodstream. After this lesson, you will be able to: ¢ Describe the role of the lymphatic system in maintaining fluid homeostasis. e Explain the immunological functions of the lymphatic system and identify how the lymphatic system contributes to pathogen surveillance. ¢ Describe the absorption and transport of dietary lipids. e Identify the composition of lymph with plasma and interstitial fluid. Which of the following substances is hydrophobic and cannot be easily transported in blood? Potassium ions Monosaccharides Albumin Chloride ions (v] Triglycerides Which vessels are responsible for reabsorption of fluid from the interstitial space? Venules (v} Lymphatic capillaries Arteries Arterioles Veins Which leukocytes play an essential role in adaptive immunity, identifying and combating infected or abnormal cells? Eosinophils @ Lymphocytes Basophils Erythrocytes Neutrophils ted system that co of essels, organs, and tissues that penetrate almost every tissue of the body. The different lymphatic organs include the thymus, tonsils, red bone marrow, spleen, and lymph nodes. These organs are densely populated with lymphocytes that will fight against pathogens and activate an immune response. Lymphocytes are produced in the red bone marrow. We will look at the lymphati rgans, tissues, and vessels in the next concept. Before we dive into those specific components, it is important to be familiar with the primary roles of this body system. The lymphatic system has three functions: * fluid recovery + immunity * lipid absorption Immunity Along with the excess tissue fluid that is recovered, the lymphatic system will also pick up any chemicals or foreign cells from tissues. The lymph (recovered fluid) will pass through lymphatic vessels and lymph nodes that contain immune cells like lymphocytes that will seek out and destroy any pathogens, abnormal cells, or potentially harmful foreign substances. This ensures that the lymphatic fluid that enters the bloodstream is free of pathogens that could spread throughout the body. Lipid Absorption The lymphatic system is responsible for absorbing dietary fats and fat-soluble vitamins from the digestive system. The small intestine contains special lymphatic vessels called lacteals that will absorb these nutrients and transport them to the bloodstream via lymph. Dietary lipids are too large to be transported through blood capillaries and enter the lacteals as a result. Since these lipids are hydrophobic, they will combine with transport proteins (lipoproteins) that will help transport them through the lymphatic fluid and blood plasma, which are both composed mostly of water. Pulmonary capillaries Systemic capillaries Approximately what percentage of interstitial fluid is reabsorbed by blood capillaries? 5% 25% 15% @ 35% How does lymph differ from blood? iv) Lymph has a lower protein content. Lymph flows in closed circulatory systems. Lymph contains more red blood cells. Lymph is always thicker than blood. Let's recap the main functions of the lymphatic system before we dive deeper into the different organs and vessels. e The lymphatic system recovers excess fluid from tissues that the blood capillaries do not filter back into the blood. This recovered fluid is called lymph and eventually gets put into the blood. e As lymph is recovered, it passes through vessels and lymph nodes that contain white blood cells that will detect and remove any pathogens or foreign substances. Lymphocytes are the main immune cells that help with this. e Lymphatic vessels called lacteals are found in the small intestine. These specialized vessels absorb dietary lipids that are too large to be transported through the blood and transport them via lymph. e Lymph fluid is a colorless, clear fluid that is similar to blood plasma but with a lower protein concentration. e Lymph fluid also contains bacteria, cellular debris, and other substances that will be filtered en route to the bloodstream. Which of the following are the lymphatic organs? Select all that apply. Thyroid Kidneys Lymph nodes Red bone marrow Small intestine Thymus Failure to return excess fluid to the bloodstream via the lymphatic system could result in which condition characterized by tissue swelling? Diabetes Circulatory shock © Edema Hypotension Atherosclerosis Which cells are densely populated in lymph nodes and activate an immune response against foreign matter or pathogens? Erythrocytes Thrombocytes ©@ Lymphocytes Myocytes Osteacytes What is the primary purpose of the lymphatic system's fluid recovery function? To produce antibodies To regulate body temperature To facilitate digestion @ To maintain proper fluid balance What happens to excess interstitial fluid that is not reabsorbed by blood capillaries? It becomes urine and is excreted by the kidneys. (v} It is transported by lymphatic vessels as lymph. It is stored in the liver for later use. It is absorbed by the skin. Where are lymphocytes produced? Spleen Thyroid Tonsils Cv] Red bone marrow Thymus The lymphatic system consists of a series of vessels that carry the fluid lymph from tissues to the subclavian veins. These vessels, along with different lymphatic organs, help maintain fluid balance and provide immune defense. In this topic, we will talk about the pathway of lymph through the lymphatic vessels, and discuss the structure and function of different lymphatic organs. After this lesson, you will be able to: = Identify the primary and secondary lymphatic structures, including the bone marrow and thymus as well as lymph nodes, spleen, tonsils, MALT and Peyer's patches. * Differentiate between lymphatic capillaries, collecting vessels, trunks, and ducts. + Trace the flow of lymph from peripheral tissues through lymphatic vessels to its return into the venous circulation. LYMPHATIC SYSTEM. Lymphatic system Many organs such as the adrenal glands, brain, and kidneys contain an outer tissue region known as the cortex and an inner region called the The fluid that we will focus on in our discussion of the lymphatic system is lymph. Recall that lymph is the clear, colorless recovered fluid. This excess fluid may also contain macrophages, viruses, bacteria, cellular debris, and hormones that were taken up from the tissues. This fluid will travel through a system of lymphatic vessels that will carry lymph to the bloodstream. The lymphatic vessels lack a pump, such as the heart, and rely on the stretching of the vessels due to an increase of lymph to open valves that allow unidirectional flow of the lymph. Much like venous return, lymphatic flow can be produced by skeletal muscle pumps and, due to their proximity to many arteries, the pressure within those arteries can drive lymph circulation. The lymphatic vessels are similar to blood vessels except that they are closed at one end, provide a one-diraction flow for fluid, and their cells are not joined by tight junctions. These vessels penetrate nearly every tissue of the body except for bone, bone marrow, cartilage, and the cornea. Lymphatic vessel pressure in the tis n thin endothelial cells that overlap like the shingles of a roof that act as valves that open and close. When the it pushes the valve-like structure and fluid flows inward. When pressure is higher in the lymphatic vessel than it isin the tissue, the valves are closed The smallest lymphatic vessels that first collect lymph are known as lymphatic capillaries. As the lymphatic capillaries converge along their path, they become larger vessels called collecting vessels. These will empty lymph into the lymph nodes to be filtered. Once lymph leaves the lymph node from the other side, it will continue traveling down the collecting vessels, often encountering additional lymph nodes. The collecting vessels converge to form lymphatic trunks that drain a major portion of the body. These are named based on the part of the body they drain. The lymphatic trunks include paired lumbar, bronchomediastinal, subclavian, and jugular trunks, and a solitary intestinal trunk. The lumbar and intestinal trunks form a sac called the cisterna chyli. The lymphatic trunks then converge to form the largest vessels known as collecting ducts. These include the right lymphatic duct and the thoracic duct. ¢ The right lymphatic duct receives lymph from the right arm, right thorax, and right head. This large vessel will empty the lymph into the bloodstream via the right subclavian vein. e The thoracic duct, which is larger and longer, receives lymph from everywhere else—the left thorax, left arm, left head, and all the body below the diaphragm. This duct empties into the left subclavian vein. Let's recap the route from the tissue fluid back into the bloodstream: Interstitial fluid — lymphatic capillaries — collecting vessels —- lymphatic trunks —> collecting ducts — subclavian veins What is the next step in the pathway of lymph drainage after lymphatic capillaries? (v) Collecting vessels Lymphatic trunks Lymph nodes Collecting ducts With a basic understanding of the lymphatic vessels and the flow of lymph, we can take a closer look at the major structures associated with the lymphatic system. Lymphatic tissue consists of groups of lymphocytes found in connective tissues of mucous membranes and various organs. The most prevalent form of lymphatic tissue occurs in a diffuse manner, scattered throughout the body, especially in passages that are open to the exterior. These include the respiratory, digestive, urinary, and reproductive tracts. This lymphatic tissue is called mucosa-associated lymphatic tissue (MALT). In some places, monocytes and lymphocytes congregate into dense masses called lymphatic nodules (follicles). Lymphatic nodules are masses of lymphatic tissue that are not surrounded by a capsule. These are essentially temporary clusters that form wher pathogens invade tissues, primarily in the mucous membranes of previously listed tracts. An example would be Peyer’s patches found in the ileum of the small intestines. Villus Blood vessels Lymphatic vessels ==_Interactin.3D (+) a ie Muscularis externa Lymphatic organs are non-moving structures that consist of a connective tissue capsule that separates the lymphatic tissue from the neighboring tissues. Examples include red bone marrow, lymph nodes, tonsils, thymus, and the spleen. Primary lymphatic organs are the organs directly involved in the production and maturation of lymphocytes. These include the red bone marrow and thymus. Secondary lymphatic organs include the tonsils, spleen, and lymph nodes. These are the places where mature lymphocytes migrate to play a role in immune responses. Which organs are considered primary lymphatic organs? Select all that apply. Thymus Tonsils Red bone marrow Spleen Lymph nodes Red bone marrow is a soft, loosely organized substance found primarily in flat bones such as the pelvis, ribs, skull, vertebrae, and epiphyses of the femur and humerus. This substance produces all formed elements of blood including red blood cells, platelets, and white blood cells. An abundance of small arteries penetrates through bone tissue and empties into sinusoids in red bone marrow. The spaces between the sinusoids consist of hematopoietic tissues, macrophages, and blood cells in different developmental stages. Red bone marrow occupies the medullary cavity of nearly all bones in children but is limited to the listed bones above in adulthood. This type of bone marrow can be aspirated from bones for a transfusion or biopsy. Disorders related to red bone marrow include leukemia, aplastic anemia, and myelodysplastic syndromes, which can all lead to abnormalities in blood cell production and function. Bone Marrow Function ‘The thymus is a vital organ that plays a role in the endocrine, immune, and lymphatic systems. This small, glandular organ is found within the mediastinum above the heart. It is most active during childhood and adolescence, and gradually degenerates in size and function through age. This process is known as involution, and the glandular tissue is replaced with adipose tissue. Like lymph nodes, the thymus is enclosed in a capsule that divides the thymic tissue into lobules. Each lobule has an outer region called the cortex that contains immature T-cells and an inner region known as the medulla, As the cortex is surrounded by blood capillaries, T lymphocytes that are produced in red bone marrow will isolate there and develop. They will then migrate to the medulla of the lobule and mature for another three weeks under the influence of thymic hormones (e.g., thymosin, thymopoietin, and thymulin). 
