Chapter 22 The Lymphatic System BIO 211 Lab Instructor Dr. Gollwitzer 1

Today in class we will discuss: – Functions of the lymphatic system – Identify the major components of the lymphoid system – Describe the structure and distribution of lymphatic vessels – Trace lymph flow from peripheral tissues to the heart 2

Functions of the Lymphatic System Produces, maintains, and distributes lymphocytes (primary cells of the lymphatic system) Returns fluids and solutes from the peripheral tissues to circulation and maintains volume Distributes nutrients, waste products and hormones from tissues of origin to general circulation – e.g., lipids absorbed by GI tract cannot enter capillaries; enter lymphatics (lacteals) to return to circulation 3

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Components/Organization Lymphocytes = primary cells Lymphoid tissues = CT dominated by lymphocytes – Lymphoid nodule = densely packed lymphocytes in areolar tissue; no fibrous capsule Lymphoid organs = connected to lymphatic vessels and have a capsule – Nodes – filter and purify lymph and provide early warning system – Thymus – produces mature T cells – Spleen – filters and purifies blood Network of lymphatic vessels (lymphatics) = distribution system that begins in peripheral tissues and ends at veins Lymph = fluid connective tissue similar to plasma, but with no plasma proteins 5

Lymphocytes 20-30% of circulating WBCs Most of lymphocytes are not circulating in blood; found in tissues and organs Respond to presence of – Invading pathogens (bacteria, viruses…) – Abnormal body cells (virus-infected cells or cancer cells) – Foreign proteins (toxins released from bacteria) Attempt to eliminate these threats or make them harmless by a combination of physical and chemical attacks 6

Figure 19–10 Types of WBCs 7

Lymphocytes Produced by lymphopoiesis in – Red bone marrow (initial site) – Lymphoid tissues (e.g., tonsils) – Lymphoid organs (e.g., thymus) Derived from hemocytoblasts, lymphoid stem cells Stored in lymphoid tissues and organs and red bone marrow 8

Figure 22-6 Lymphopoiesis 9

3 Main Types of Lymphocytes T cells – Thymus-dependent B cells – Bone-marrow derived NK cells – Natural killer cells 10

Types of Lymphocytes T cells – 80% of circulating lymphocytes – Cytotoxic T cells Attack foreign cells or cells infected by viruses Often involves direct contact Primary cells involved in CMI (cell-mediated immunity) – Helper T cells Stimulate responses of T and B cells B cells must be activated by helper T cells before they can produce antibodies – Suppressor T cells Inhibit T and B cell activites Modify the immune response 11

Types of Lymphocytes B cells – 15% of circulating lymphocytes – Differentiate into plasma cells  antibodies – Antibody + target antigen  destruction of target compound or organism – Anti-body-mediated immunity or humoral (liquid) immunity Natural killer (NK) cells - 5% of circulating lymphocytes – Police of peripheral tissues = immunological surveillance – Directly attack Foreign cells Cells infected with viruses Cancer cells 12

Lymphoid Tissues CT dominated by lymphocytes Form lymphoid/lymphatic nodules – Densely packed lymphocytes in areolar tissue – No capsule – Have germinal center where lymphocytes are dividing – Found in CT deep to epithelia lining respiratory, digestive, and urinary tracts 13

Lymphoid Tissues Include – Tonsils = 5 large nodules in wall of pharynx 1 pharyngeal tonsil (adenoids) – in posterior superior wall of nasopharynx 2 palatine tonsils – at posterior margin of oral cavity, along boundary with pharynx 2 lingual tonsils – under tongue – Vermiform appendix Blind pouch that contains fused lymphoid nodules Originates near junction between small and large intestines – MALT (mucosa-associated lymphoid tissues) Collection of lymphoid tissues linked with digestive system e.g., Peyer’s Patches = clusters of lymphoid nodules deep to lining of intestines 14

Figure 22-7b 15

Figure 22-7a 16

Lymphoid Organs Tissue + capsule Distinguishing characteristic – Fibrous CT capsule separates organs from surrounding tissues Include – Lymph nodes – Thymus – Spleen 17

Lymph Nodes Filter/purify lymph Small (ca 1 in. max), kidney bean-shaped organs Anatomy – Capsule - tough, fibrous CT that separates organ from surrounding tissues – Trabeculae - bundles of collagen fibers that extend from the capsule and form partitions/walls – Hilus - shallow indentation where vessels and nerves enter/leave 18

Lymph Nodes Anatomy (continued) – Cortex - outer (B cells) and deep (T cells) Germinal centers = central zones in the cortex of a lymphoid nodule that contain dividing lymphocytes – Medulla - core; contains B cells and plasma cells organized into medullary cords – Sinuses - open passageways with incomplete walls – Other cells: dendritic dells = “parenchymal” cells that bind antigens; involved in initiation of immune response – Lymph node vessels Afferent vessels - carry lymph to node from peripheral tissues Efferent vessel - carries lymph away from node toward venous circulation at hilus 19

Fig. 22-8, part 2, 9 th edition 20 Cortical sinus

Figure. 22-7, part 1, 7 th edition 21

Lymph Flow through Lymph Node Occurs through network of sinuses From afferent lymphatics, enters subcapsular sinus that contains a meshwork of – Branching reticular fibers – Macrophages – Dendritic cells = antigen-presenting cells; involved in initiation of immune response 22

Lymph Flow through Lymph Node To cortex – Outer cortex contains B cells with germinal centers (like lymphoid nodules) – Deep cortex dominated by T cells; lymphocytes leave blood stream and enter lymph nodes here To medulla/medullary sinus – Core of lymph node – Contains B cells and plasma cells organized into medullary cords Exits via efferent lymphatic at hilus 23

Lymph Nodes Aggregations of lymph nodes (“lymph glands” a misnomer) Located where peripheral lymphatics connect with trunk – Axillary – Inguinal – Cervical “Swollen glands” – Indicate inflammation or infection in peripheral tissues – Painful enlargement due to increase in number of lymphocytes and phagocytes in response to minor, localized infection – Nonpainful, chronic or excessive enlargement = lymphadenopathy May occur in response to bacterial or viral infections, endocrine disorders, cancer 24

Functions of Lymph Nodes Filter and purify lymph before entering venous circulation Fixed macrophages in walls of sinuses engulf debris and pathogens Remove 99% of antigens – Some processed by macrophages and presented to nearby lymphocytes – Others bind to dendritic cells where they can stimulate lymphocyte activity= antigen presentation = first step in immune response Provide an early warning system – Infection or abnormality in peripheral tissue  abnormal antigens into interstitial fluid  lymph  lymph nodes  stimulate macrophages and lymphocytes 25

Thymus Produces mature T cells Location in mediastinum, posterior to sternum Appearance is pink with a grainy consistency Changes over lifespan – Largest size (relative to body weight) in first 2 years of life – Maximum size just before puberty – After puberty, begins to involute (get smaller) – Continues to decrease in size and becomes fibrous; may be cause of decreased immunity in older population 26

Figure 22-9 The Thymus Thyroid gland Trachea Right lobe Diaphragm Right lung THYMUS Lymphocytes The appearance and position of the thymus in relation to other organs in the chest. Left lobe Heart Right lobe Left lobe Cortex Septa Medulla Lobule Anatomical landmarks on the thymus. Septa Lobule The thymus gland LM  50 Fibrous septa divide the tissue of the thymus into lobules resembling interconnected lymphoid nodules. A thymic corpuscle LM  550 Higher magnification reveals the unusual structure of thymic corpuscles. The small cells are lymphocytes in various stages of development. Thymic corpuscle Reticular cells Left lung 27

Thymus Anatomy – Has a capsule – 2 connected lobes – Septae (plural) Fibrous partitions; divide lobes into lobules – Lobules Cortex - densely packed cells, including – Dividing lymphocytes, arranged in clusters surrounded by… – Reticular epithelial cells » Maintain blood-thymus barrier » Secrete thymic hormones (thymosins) that stimulate stem cell divisions and T cell differentiation into mature T cells » Mature T cells then migrate to medulla Medulla - no barrier so cells can now enter blood 28

Spleen Filters/purifies blood Largest collection of lymphoid tissue Functions – Removes abnormal blood cells and other blood components by phagocytosis – Stores iron from recycled RBCs – Initiates immune responses by B cells and T cells in response to antigens in circulating blood 29

Spleen Anatomy – Soft consistency; reflects shape of organs that surround it (stomach, left kidney, diaphragm) – Fibrous capsule – Hilus (groove, blood & lymphatic vessels enter/exit) – Red pulp - large sinusoids containing many RBCs plus macrophages and lymphocytes – White pulp - dense lymphocytes  white appearance; resembles lymphoid nodules; can respond to any antigen in blood 30

Spleen Impact to left side in automobile accidents, contact sports…spleen can tear easily  internal bleeding and circulatory shock – Due to fragility, difficult to repair surgically – Usually remove entire organ (splenectomy) – Person survives without difficulty but at risk for bacterial infections 31

Figure 22-10, 7 th edition 32

Lymphatic Vessels From peripheral tissues to venous system Lymphatic capillaries – “Terminal” lymphatics; misnomer because mark beginning of lymphatic vessels – Branches in peripheral tissues – Different from regular capillaries Originate as blind pockets Larger in diameter Thinner walls In cross-section- have flattened/irregular outline Lined by endothelium, with incomplete/absent basement membrane 33

Lymphatic Vessels Lymphatic capillaries (continued) – Endothelial cells not tightly bound, but overlap Acts as 1-way valve, propels lymph in one direction Permits entry of large solutes, e.g., proteins, bacteria, viruses, cell debris, but prevents their return to intercellular spaces – Present in almost every tissue and organ (except cornea, bone marrow, CNS) – Lacteals Special lymphatic capillaries in small intestine Transport lipids from digestive tract 34

Figure

Lymphatic Vessels Small lymphatic vessels – Lead toward trunk (via larger lymphatics) – Similar to veins Same vessel walls Minimal pressures Have valves – Prevent backflow Often in close association with blood vessels – Differ in relative size (smaller), general appearance (e.g. color), and branching pattern 36

Figure 22-3a, b 37

Lymphatic Vessels Larger lymphatic vessels (2 kinds) – Superficial lymphatics Found underlying membranes throughout body (e.g., skin; mucus, serous) – Deep lymphatics Largest vessels Accompany deeper arteries and veins 38

Lymphatic Vessels Larger lymphatics converge to form large lymphatic trunks (R&L) – Jugular trunks Drain head, neck – Subclavian trunks Drain upper limbs – Bronchomediastinal trunks Drain thoracic tissues/organs – Lumbar trunks Drain back, lower limbs – Intestinal trunk Drain abdominopelvic area 39

Figure 22-4b The Relationship between the Lymphatic Ducts and the Venous System Right internal jugular vein Right jugular trunk Right lymphatic duct Right subclavian trunk Right subclavian vein Right bronchomediastinal trunk Superior vena cava (cut) Azygos vein Rib (cut) Brachiocephalic veins The thoracic duct empties into the left subclavian vein. The right lymphatic duct drains into the right subclavian vein. Left internal jugular vein Left jugular trunk Thoracic duct Left subclavian trunk Left bronchomediastinal trunk Left subclavian vein First rib (cut) Highest intercostal vein Thoracic duct Thoracic lymph nodes Hemiazygos vein Parietal pleura (cut) Diaphragm Cisterna chyli Intestinal trunk Left lumbar trunk Inferior vena cava (cut) Right lumbar trunk 40

Lymphatic Vessels Lymphatic trunks empty into two large collecting vessels – Thoracic duct Collects lymph from tissues – From L side of upper body (above diaphragm) – Inferior to the diaphragm – R lymphatic duct Collects lymph from tissues – From R side of upper body (above diaphragm) 41

Figure 22-4a, 7 th edition 42

Lymphatic Vessels Thoracic duct – Base = expanded chamber = cisterna chyli – Receives lymph from Inferior abdomen, pelvis, and lower limbs – via R and L lumbar trunks and intestinal trunks Collects lymph from L bronchomediastinal trunk, L subclavian trunk, and L jugular trunk Empties into L subclavian vein Right lymphatic duct Formed by merging of R jugular, R subclavian, and R bronchomediastinal trunks Empties into R subclavian vein 43

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Lymph Fluid in lymphatic vessels Formed from blood plasma forced out of capillary by hydrostatic pressure into interstitial spaces (  interstitial fluid) Interstitial fluid accumulates and increases pressure Lymphatics pick up excess so edema doesn’t occur As lymph returns to CVS, cells of immune system (e.g. lymphocytes, macrophages in lymphoid tissues/organs) monitor its composition and respond to signs of injury or infection Cells – 99% lymphocytes (T and B cells) – Remaining 1% are primarily phagocytes = microphages (eosinophils, neutrophils) and macrophages (monocytes) 45

Lymphedema Blockage of lymph drainage from a limb (e.g., following removal of lymph nodes) – Causes severe swelling – Interferes with immune system function 46