2023 HUB105 Lec 11 Lymphatic System PDF

Lecture 11 Lymphatic system, lymphatic vessels, tissues and organs. BChD I HUB 105 2023 Dept. Medical Biosciences University of the Western Cape  Introduction Lymphatic system (LS) forms a network of tissues, cells and...

Lecture 11 Lymphatic system, lymphatic vessels, tissues and organs. BChD I HUB 105 2023 Dept. Medical Biosciences University of the Western Cape  Introduction Lymphatic system (LS) forms a network of tissues, cells and organs responsible for defending the body against various external pathogens (environment) and internal threats, eg cancer cells. Pathogens are any microscopic organisms that cause diseases eg virus, bacteria, fungi. Each pathogen has a different life cycle and attacks the body in specific ways eg viruses are found within cells which they destroy, larger parasites will burrow into internal organs and somatic cells can turn cancerous. Components of the LS Lymph Lymphocyte Lymphatic Vessels and Lymph Nodes Cervical lymph nodes Thoracic duct Right lymphatic duct Lymphoid Tissues and Organs Axillary lymph nodes Lymphatics of Tonsil mammary gland Thymus Cisterna chyli Spleen Lymphatics of upper limb Mucosa-associated lymphoid tissue (MALT) in digestive, Lumbar lymph nodes respiratory, urinary, and reproductive tracts Figure 1 An Overview of the Lymphatic System (Part 1 of 2)  Components of the LS 1. Lymphatic vessels Carry lymph from peripheral tissues to the venous system - small vessels are called lymphatic capillaries, large vessels are called lymphatic trunks and the largest vessel is the thoracic duct - all vessels combined often called “lymphatics ‘’ 2. Lymph A clear slightly yellowish fluid that flows in lymph vessels - Derived from blood and resembles plasma but has lower protein content. - Lymph found in digestive tract contains mostly fats and appears milky in colour (triglycerides). 3. Lymphocytes specialized cells eg T- cells (thymus-dependent) , B-cells (bone marrow-derived ) and natural killer (NK) cells Other cells that perform specific functions in defending the body - eg phagocytes, macrophage and reticular cells.  Components of the LS 4. Lymphoid tissues (LT) Scattered thru out the body - are collections of loose connective tissue & lymphocytes in structures called lymphoid nodules - each nodule often has a central zone called a germinal center, which contains dividing lymphocytes - nodules cluster together to form e.g. the tonsils, Peyer’s patches & mucosa-associated LT (MALT) in the intestinal wall. The walls of the appendix also contain a mass of fused lymphoid nodules. 5. Lymphoid organs Are more complex structures that contain large numbers of lymphocytes and are connected to lymphatic vessels. - e.g. lymph nodes, spleen & thymus - there are about 1 trillion lymphocytes - with a combined weight of over 1kg within lymphoid organs & tissues! Lymphatic Cells Macrophage Reticular cells on reticular fibers Lymphocytes Reticular fiber  Functions of the Lymphatic system 1. Maintenance of the fluid balance in the internal environment - Tissues are surrounded by interstitial fluid (IF) & all interchanges of nutrients and waste products between the blood and tissue occurs across the medium of the IF. - Interstitial fluid not absorbed by the cells or reabsorbed by the capillaries enters lymphatic capillaries as lymph. - This helps prevent edemas (swelling caused due to excess fluid trapped in tissues or cavities) , the lymphatic vessels act as “drains” to collect the excess IF and return it to the venous circulation, therefore maintaining blood volume.  Functions of the Lymphatic system 3. Fat absorption - Absorption of triglycerides and other substances from the digestive tract. - Lymphatic vessels called lacteals are located in the lining of the small intestine. - Fats are absorbed into lacteals and pass through the lymphatic vessels to the venous circulation. 4. Defense (Immunity) Responsible for the production, maintenance and distribution of lymphocytes Three classes of lymphocytes in the bld are: T (thymus-dependent) cells, B (bone-marrow-derived) cells and NK(natural killer) cells. Lymphocytes are vital to the body’s ability to resist/overcome infection & disease = IMMUNITY  Lymphatic Vessels Smallest Lymphatic capillary the largest vessels: lymph To small Thoracic duct flows lymphatic & Right vessels Lymphatic toward duct the heart Superficial & Deep Larger lymphatics Lymphatic the large and Trunks Major Lymph- collecting vessels Fig 3: Lymph flow through lymphatic system from smallest vessel through to lymphatic ducts and lymph drains into veins.  Lymphatic Vessels  Carry lymph away from tissues via Smooth muscle Arteriole Endothelial Lymphatic cells capillary the smallest lymphatic vessels = Lymphatic capillaries  lymph is partly formed through the movement of fluids and solutes from plasma in blood vessels thru the interstitial fluid and into the lymphatic capillaries Lymphatic capillaries (terminal lymphatics) branch into peripheral tissues to join the larger lymphatic Interstitial Venule trunks to drain into the venous fluid circulation. Lymph flow Fig 4: The interwoven network formed by blood capillaries Blood capillaries Loose connective tissue and lymphatic capillaries © 2012 Pearson Education, Inc  Lymphatic Vessels Lymphatic capillaries Structure: Blind-ending tubes, surrounded by overlapping endothelial cells (not tightly bound) with incomplete basement membrane. Overlapping cells allows for one- way entry of fluids, solutes, bacteria and viruses. Vessels have large number of semilunar valves which permit the Fig 5: Structure of unidirectional flow of lymph Lymphatic capillary showing towards the heart. blind-ending vessels with Since most of the lymph flow overlapping occurs against gravity (“uphill”), endothelial cells these valves are very important in preventing the backflow of fluid.  Lymphatic Vessels Vein Artery Artery Vein Lymphatic Lymphatic vessel vessel Lymphatic Toward valve venous system From lymphatic capillaries Lymphatic vessel Fig 6: Connective tiss. containing small bld vessels Lymphatic and lymphatic vessels, C/S valve view indicating structural differences between vessels © 2012 Pearson Education, Inc.  Lymphatic Vessels Right Left Lymph nodes (lymph organ): subclavian subclavian vein vein Distributed along vessels and filter lymph Lymphatics include: – Lymphatic capillaries – Lymphatic collecting vessels – Lymphatic trunks and ducts the lymphatic trunks in turn empty into two large collecting ducts: the Thoracic duct and the Right lymphatic duct. Fig 7: Lymph collected and empting from lymphatic trunks into the lymphatic ducts and finally into the subclavian veins  Lymphatic Vessels Thoracic duct collects lymph from the whole body inferior to the diaphragm & the left side of the body superior to the diaphragm drains into the left subclavian vein Right lymphatic duct collects lymph from the right side of the body superior to the diaphragm drains into the right subclavian vein  Lymphatic Vessels Fig 8: The Relationship between the Lymphatic Ducts and the Venous System http://www.slideshare.net/rajud521/l-nervous-system-3395290  Lymphoid tissue (LT)  Specialized connective tissue composed primarily of lymphocytes with macrophages, dendritic cells and reticular cells (reticular fibers)  Provides a proliferation site for lymphocytes  LT is either encapsulated by a fibrous connective tissue capsule or non-encapsulated tissue Two main types of LT 1. Lymphoid nodules: - are solid, spherical bodies of tightly packed reticular elements and cells with a Germinal center with dividing lymphocytes - Located: tonsils, epithelial lining of intestine (Peyer’s patches), appendix 2. Diffuse lymphatic tissue: - tissue scattered with reticular cells and lymphocytes with no clear boundaries - Located: deep in mucous membranes, around lymphoid nodules, spleen and lymph nodes Fig 6: Lymphoid tissue  Lymphoid tissue (LT)  Collections of LT found in digestive system is referred to as Mucosa- associated lymphoid tissue (MALT)  Clusters of lymphoid nodules found deep in epithelial lining of intestine know as aggregated lymphoid nodules (Peyer’s patches) Intestinal lumen Aggregated lymphoid Nodule (Peyer’s patches) Underlying connective tissue Fig 10: Pharyngeal tonsil with lymphoid nodules Fig 9: Aggregated lymphoid nodules in large intestine  Lymphoid tissue :Tonsils tonsils are masses of lymphoid tissue forming a protective ring (pharyngeal lymphoid ring) under the mucous membranes of the mouth & back of the throat there are 5 tonsils: 1. paired palatine tonsils, located on each side of the throat 2. a single pharyngeal tonsil, near the posterior opening of the nasal cavity called the adenoid if enlarged/swollen 3. paired lingual tonsils, located at the base of the tongue function: tonsils provide protection against bacteria & other potentially harmful material entering the pharynx from the oral or nasal cavities i.e they are the first line of defense against infection of the pharyngeal wall  Lymphoid tissue :Tonsils Fig 12 A) location of tonsils and B) whitish coating indicates infected tonsils, Tonsillitis  Lymphoid Organs  Lymph nodes  Thymus  Spleen Lymph nodes Functions - Filtration of lymph removes 99% of antigens in lymph before it returns to venous circulations by macrophages that engulf and destroy pathogens and debris. - Immune response: distribution pattern of lymph nodes allows for early warning signals and lymphocytes are activated and mount an attack against antigens. Lymph nodes Lymph vessel  Lymphoid Organs: Lymph node Structure of lymph nodes lymph nodes are small, round/oval or kidney bean-shaped structures ranging from 1-25 mm in diameter & distributed along the course of lymphatic vessels a dense connective tissue capsule surrounds each lymph node and extensions of the capsule, called trabeculae, form the internal skeleton and separate the cortical nodules from one another internally, the lymph nodes can be divided into a cortex and medulla the cortex (outer & deep) contains cortical nodules within sinuses - sinuses contain a meshwork of branching reticular fibers, macrophages & dendritic cells (= cells involved in initiating the immune response) - the outer cortex of the nodule is packed with B-lymphocytes & germinal centers contain dividing, immature B-cells the medulla is composed of medullary sinuses and medullary cords, both which are lined with B-lymphoctyes & macrophages that can perform phagocytosis Lymphoid Organs: Lymph node  Lymphoid Organs: Lymph node Flow of lymph through the node lymph enters the lymph node through several afferent lymphatic vessels and leaves the node through one efferent lymphatic vessel via the hilus. once lymph enters the node, it slowly flows through the sinuses where it encounters macrophages & dendritic cells then the lymph flows through the outer cortex of the node, which is dominated by B-lymphocytes lymph then continues through lymph sinuses in the deep cortex, which is dominated by T-lymphocytes. At this point, circulating lymphocytes leave the bloodstream and enter deep cortex lymph then continues into the medulla, where it passes through medullary sinuses before draining into the efferent vessel unidirectional valves in both afferent and efferent vessel keep lymph flowing in one direction  Lymph is propelled thru lymphatic system: Pulsations of nearby arteries and contractions of smooth muscle in the walls of the lymphatics helps lymph to flow  Lymphoid Organs: Thymus  the thymus is a pink gland located in the mediastinum, posterior to the sternum, whose size gradually decreases after puberty  Function - Plays vital role in immunity mechanism, site for T cells (Thymus lymphocyte) differentiation and maturation. - Secretes several thymic hormones, grp name thymosins and other regulators, which enables lymphocytes to develop into T cells  Lymphoid Organs: Thymus Structure of the Thymus - it consist of two pyramidal lobes that are subdivided into small lobules by fibrous septae, each with a densely packed outer cortex and a paler, central medulla - Lymphocytes divide in the cortex while T cells mature and migrate into medulla ; leave thymus by medullary blood vessels. - Within the cortex hormone-secreting cells surrounding the clusters of lymphocytes release thymosins, which stimulate lymphocyte stem cell division, T cell maturation and maintain immunological defenses  Lymphoid Organs: Spleen - the spleen is located in the superior part of the abdominal cavity, wedged between the stomach, left kidney and diaphragm - the spleen contains the largest collection of lymphoid tissue in the body Functions - its function is similar to lymph nodes except that instead of filtering lymph, it filters blood; hence its deep red colour - Removal of abnormal blood cells and other blood components by phagocytosis - Storage of iron recycled from red blood cells - Initiation of immune responses by B cells and T cells in response to antigens in circulating blood Diaphragm Spleen Adrenal gland Left kidney Splenic artery Pancreas  Lymphoid Organs: Spleen  Structure: Spleen - Ovoid in shape - Surrounded by fibrous capsule with inward extensions that divide the organ into compartments - Internally the spleen also resembles lymph nodes: it is divided into compartments, which consist of - “white pulp” = an area resembling lymphoid nodules that contains developing lymphocytes - “red pulp” = a network of fine reticular fibers submerged in blood - Splenic artery enters at hilus and branches to form finer Trabecular Arteries - Lymphocytes are found thru out red pulp; white pulp has high content of macrophages. Lymphoid Organs: Spleen White pulp of splenic nodule Capsule Red pulp Trabecular artery Central artery in splenic nodule Revision Questions: 1. List and briefly describe the components of the lymphoid system. 2. Explain the flow of lymph through the lymphatic systems. 3. Name the functions of the lymphatic system. 4. Explain the formation of lymph. 5. Describe the structure of lymphatic tissue. 6. Lymph circulates through the lymph nodes, explain how and why this occurs. 7. Describe the structure of the lymphatic vessels. 8. Describe the structure of the Thymus and Spleen. 9. Explain the function of the thymus. 10.Explain the function of the spleen.