A&P II Chapter 20: The Lymphatic System .

General information about the lymphatic system 

• Three parts of the lymphatic system 

• Network of lymphatic vessels (lymphatics)/ the lymph organs  

• Lymph: fluid in vessels/ the lymph organs  

• Lymph nodes: cleanse lymph 

• Lymphoid tissue – primary functions 

• Provide structural basis of immune system by housing phagocytic cells and lymphocytes 

• Lymphatic vessels 

• Directionality of fluid flow 

• One-way system, flow only towards the heart 

• Where/what vessels drain into the venous circulation 

• Right lymphatic duct drains right upper arm and right side of head and thorax 

• Thoracic duct drains rest of body 

• Lymphatic capillaries 

• Structure 

• Blind-ended vessels that weave between tissue and cells and blood capillaries.  

• They are more preamble than blood capillaries 

• Compare/contrast their structure and function to capillaries, arteries, and veins 

• Capillaries: Exchange materials; lymphatic capillaries absorb excess interstitial fluid. 

• Arteries: carry blood away from the heart; lymphatic vessels carry fluid toward the heart. 

• Veins: Carry deoxygenated blood; lymphatic vessels use similar mechanisms (valves, muscle contraction) to return fluid. 

• Lymph transport 

• Compare it to transport in the venous system 

• It is the same low-pressure system as the venous system  

• Mechanisms that help propel lymph 

• Milking action of skeletal muscle 

•  Pressure changes in thorax during breathing 

• Valves to prevent backflow 

•  Pulsations of nearby arteries 

• Contractions of smooth muscle in walls of lymphatics 

Primary versus secondary lymphoid organs 

• Which organs are primary, and which are secondary 

• Primary: Red bone marrow and thymus  

• Secondary:  Nodes, spleen, MALT, and diffuse lymphoid tissue 

• What occurs in primary versus secondary organs 

• Primary: areas where T and B cells mature. ( B in bone marrow, and T in thymus) 

• Secondary: areas where mature lymphocytes first encounter their antigen and become activated 

• Changes that occur in these organs as we age (only mentioned for some of the primary or secondary organs) 

• Thymus stops growing during adolescence, then gradually atrophies 

• Spleen and lymph nodes may become less efficient in older adults 

Lymph nodes 

• Purpose – what important events take place in the lymph nodes 

• The cleansing of the lymph as it flows back into the bloodstream  

• The immune system activation, which offers a place for lymphocytes to become activated and mount an attack against antigens 

• Structure 

• Vary in shape and size but most are bean-shaped 

• Small, less than 2.5 cm (~1 inch) 

• Fluid flow (circulation) in the lymph nodes 

• Step 1: The lymph enters the convex side of node through the afferent lymphatic vessels  

• Step 2: Travels through large subcapsular sinuses and then into smaller sinuses found throughout the cortex and medulla. 

• Step 3: The lymph enters the medullary sinuses 

• Step 4: It exits on the concave side at the hilum using the efferent lymphatic vessels  

• Location 

• All throughout the body but most are embedded deep in the connective tissue in cluster along the lymphatic vessels  

Spleen 

• Functions 

• Site of lymphocyte proliferation and immune surveillance and response 

• More important is its blood cleansing function 

• Cleanses aged or defective blood cells and platelets  

• Macrophage remove debris  

• Stores breakdown products of RBC’s that later recycled 

• Stores blood platelets and monocytes for release into blood when needed 

• May be site of fetal erythrocyte production  

• White pulp versus red pulp 

• White pulp 

• Site where immune functions occur 

• Contains mostly lymphocytes attached to reticular fibers  

• Red pulp 

• Site where old blood cells and bloodborne pathogens are destroyed 

• Rich in RBC and macrophages that engulf them 

• Composed of splenic cords (reticular connective tissue) that separate blood-filled splenic sinusoids (venous sinuses) 

MALT ( Mucosa-associated lymphoid tissue) 

• What is it 

• Lymphoid tissues in mucous membranes throughout the body. It provides protection from pathogens trying to enter the body.  

• Names and locations of largest MALT collections found in the body 

• Tonsils 

• Palatine tonsils: Posterior end of the oral cavity ( gets  

• Lingual tonsils: the base of the tongue 

• Pharyngeal (adenoids) tonsils: posterior wall of the nasopharynx 

• Tubal tonsil: surrounding openings of the auditory tubes into the pharynx 

• Peyer’s patches: found in the distal part of the small intestine 

• Appendix: found in the first part of the large intestine 

• MALT patches in the head region have specific names, be familiar with their names and clinically relevant information presented in the notes 

• Inflamed tonsils= tonsillitis 

• Infection of appendix= appendicitis  

• Tonsillar crypts- small pocket or crevices in the trap bacteria and other foreign substances. They are part of the immune system and help the body fight infection  

Thymus 

• Function 

• It is a lymphoid organ where T cell mature  

• Location 

• Lobules that contain outer cortex and inner medulla  

• How the thymus differs from other lymphoid organs 

• It has no follicles because it lacks B cells  

• Does not directly fight antigens  

• Stroma is made up of epithelial cells and does not have reticular fibers