The Lymphatic System and Lymphoid Organs and Tissues 20 The Lymphatic System and Lymphoid Organs and Tissues

Consists of three parts Lymphatic System Consists of three parts A network of lymphatic vessels (lymphatics) Lymph (fluid inside vessels) Lymph nodes (cleanse the lymph)

Lymphatic System: Functions Returns interstitial fluid and leaked plasma proteins back to the blood Carry absorbed fat from intestine to the blood (through lacteals) Once interstitial fluid enters lymphatics, it is called lymph Together with lymphoid organs and tissues, provide the structural basis of the immune system

One-way system, lymph flows toward the heart Lymphatic Vessels One-way system, lymph flows toward the heart Lymph vessels (lymphatics) include: Lymphatic capillaries Lymphatic collecting vessels Lymphatic trunks and ducts

Figure 20.1 Lymphatic system: Lymph duct Lymph trunk Lymph node capillary Blood capillaries collecting vessels, with valves Heart Arterial system Venous system Tissue fluid Tissue cell (a) Structural relationship between a capillary bed of the blood vascular system and lymphatic capillaries.

Lymphatic Capillaries Similar to blood capillaries, except Very permeable (take up cell debris, pathogens, and cancer cells) Endothelial cells overlap to form one-way minivalves, and are anchored by collagen filaments, preventing collapse of capillaries

Lymphatic Capillaries Filaments anchored to connective tissue Fibroblast in loose connective tissue Endothelial cell Flaplike minivalve (b) Lymphatic capillaries are blind-ended tubes in which adjacent endothelial cells overlap each other, forming flaplike minivalves.

Lymphatic Capillaries Absent from bones, teeth, bone marrow and the CNS Lacteals: specialized lymph capillaries present in intestinal mucosa Absorb digested fat and deliver fatty lymph (chyle) to the blood

Lymphatic Collecting Vessels Similar to veins, except Have thinner walls, with more internal valves Anastomose more frequently Superficial collecting vessels in the skin, travel with superficial veins Deep collecting vessels travel with arteries Nutrients are supplied from branching vasa vasorum

Lymph is delivered into one of two large ducts Lymphatic Ducts Lymph is delivered into one of two large ducts Right lymphatic duct drains the right upper arm and the right side of the head and thorax Thoracic duct arises from the cisterna chyli and drains the rest of the body Each empties lymph into venous circulation at the junction of the internal jugular and subclavian veins on its own side of the body

Cisterna Chyli Located anterior to L1, L2 Collects lymph from lower limbs and from the intestinal trunk that drains the digestive organs In the intestines, specialized lymphatic capillaries called lacteals that transport absorbed fat from the small intestine to the blood stream Fatty lymph is called chyle

lymphatic duct into vein Regional lymph nodes: Internal jugular vein Cervical nodes Entrance of right lymphatic duct into vein Entrance of thoracic duct into vein Axillary nodes Thoracic duct Cisterna chyli Aorta Inguinal nodes Lymphatic collecting vessels Drained by the right lymphatic duct Drained by the thoracic duct (a) General distribution of lymphatic collecting vessels and regional lymph nodes. Figure 20.2a

Internal jugular veins Esophagus Right lymphatic duct Right jugular trunk Internal jugular veins Esophagus Right lymphatic duct Right subclavian trunk Trachea Left subclavian trunk Right subclavian vein Left jugular trunk Right broncho- mediastinal trunk Left subclavian vein Brachiocephalic veins Entrance of thoracic duct into vein Superior vena cava Left bronchomediastinal trunk Azygos vein Ribs Thoracic duct Hemiazygos vein Cisterna chyli Right lumbar trunk Left lumbar trunk Inferior vena cava Intestinal trunk (b) Major lymphatic trunks and ducts in relation to veins and surrounding structures. Anterior view of thoracic and abdominal wall. Figure 20.2b

Lymph Transport Lymph is propelled by Pulsations of nearby arteries Contractions of smooth muscle in the walls of the lymphatics Skeletal muscle contraction Breathing Lymph movement is very slow and it depends on movement of adjacent tissues

Lymphoid Cells

Lymphoid Cells Lymphocytes arise in red bone marrow and mature into 1 of 2 main varieties T cells (T lymphocytes) B cells (B lymphocytes)

T cells and B cells protect against antigens Lymphocytes T cells and B cells protect against antigens Antigens are anything that the body perceives as foreign These antigens will provoke an immune response Bacteria and their toxins; viruses Mismatched RBCs or cancer cells

Lymphocytes T cells B cells Manage the immune response Attack and destroy foreign cells B cells Produce plasma cells, which secrete antibodies Antibodies bind to antigens and mark cells for destruction

Other Lymphoid Cells Macrophages phagocytize foreign substances and help activate T cells Dendritic cells capture antigens and deliver them to lymph nodes Reticular cells produce stroma that supports other cells in lymphoid organs (structural cells)

Macrophage Reticular cells on reticular fibers Lymphocytes Medullary sinus Reticular fiber Figure 20.3

Types of Lymphoid Tissue Diffuse lymphoid tissue Lymphoid follicles (or nodes)

Diffuse Lymphoid Tissue Loose arrangement of lymphoid cells and some reticular fibers Found in virtually every body organ Larger collections appear in mucous membrane areas, like the GI and respiratory tract

Principal lymphoid organs of the body Lymph Nodes Principal lymphoid organs of the body Embedded in connective tissue, in clusters along lymphatic vessels Near the body surface in inguinal, axillary, and cervical regions of the body These are clinically important, as they are palpable on PE Firm, somewhat rubbery non-fixed mass the size of a pea or larger

lymphatic duct into vein Regional lymph nodes: Internal jugular vein Cervical nodes Entrance of right lymphatic duct into vein Entrance of thoracic duct into vein Axillary nodes Thoracic duct Cisterna chyli Aorta Inguinal nodes Lymphatic collecting vessels Drained by the right lymphatic duct Drained by the thoracic duct (a) General distribution of lymphatic collecting vessels and regional lymph nodes. Figure 20.2a

Lymph Nodes Functions Filter lymph—macrophages destroy microorganisms and debris Immune system—lymphocytes are activated and mount an attack against antigens

Structure of a Lymph Node Surrounded by a dense, fibrous capsule, from which connective tissue strands called trabeculae extend inward dividing the node into compartments (follicles) 2 histologically distinct regions Cortex Superficial and deep Medulla

Structure of a Lymph Node Cortex Superficial Made up of follicles with germinal centers where B cells divide and proliferate Deep houses T cells T cells circulate continuously between the blood, lymph nodes, and lymph, performing their surveillance role

Structure of a Lymph Node Medulla Medullary cords are thin inward extensions from the cortical lymphoid tissue and contain both types of lymphocytes Lymph sinuses Large lymph capillaries where macrophages reside and phagocytize foreign matter in the lymph as it flows by in the sinuses

Circulation in the Lymph Node Lymph enters through many afferent vessels on convex side  large subcapsular sinus  smaller sinuses that cut through the cortex  medullary sinuses  exits node at hilum (indented region on concave side) with few efferent vessels

Circulation in the Lymph Node Less efferent vessels, so the flow of lymph stagnates somewhat, allowing time for lymphocytes and macrophages to work Lymph passes through several nodes before it is cleansed

(a) Longitudinal view of the internal structure of a lymph Figure 20.4a Afferent lymphatic vessels Efferent lymphatic Capsule Trabeculae Hilum Cortex • Lymphoid follicle • Germinal center • Subcapsular sinus Medulla: • Medullary cord • Medullary sinus (a) Longitudinal view of the internal structure of a lymph node and associated lymphatics

(b) Photomicrograph of part of a lymph node (72x) Follicles Trabecula Subcapsular sinus Capsule Medullary cords Medullary sinuses (b) Photomicrograph of part of a lymph node (72x) Figure 20.4b

Largest lymphoid organ, size of a fist Spleen Largest lymphoid organ, size of a fist Located in the left side of the abdominal cavity, just beneath the diaphragm, lateral to the stomach Blood supply Splenic artery and vein that enter and exit the hilum (concave anterior surface)

(c) Photograph of the spleen in its normal position Diaphragm Spleen Adrenal gland Left kidney Splenic artery Pancreas (c) Photograph of the spleen in its normal position in the abdominal cavity, anterior view. Figure 20.6c

Spleen Function Functions Site of lymphocyte proliferation and immune surveillance and response Cleanses the blood of aged cells and platelets and debris

Spleen Stores breakdown products of RBCs (e.g., iron) for later reuse Releases other breakdown products to the blood for processing in the liver Stores blood platelets and monocytes to be released when needed Contains lymphocytes, macrophages, and huge numbers of erythrocytes

Histological Structure of the Spleen Has a fibrous capsule and trabeculae Two distinct areas White pulp Red pulp

Histological Structure of Spleen White Pulp Where immune function takes place Composed mostly of lymphocytes suspended on reticular fibers White pulp forms clusters around central arteries (small branches of the splenic artery) and form what appear to be islands in a sea of red pulp

Histological Structure of the Spleen Red Pulp Where worn out RBCs and bloodborne pathogens are destroyed Contains lots of RBCs and macrophages that engulf the RBCs Consists of splenic tissue that is not white pulp (the rest of the spleen) Splenic sinuses (venous sinuses) Splenic cords (reticular CT that separates splenic sinuses)

Thymus

Thymus Size with age In infants, it is found in the inferior neck and extends into the mediastinum, where it partially overlies the heart Increases in size and is most active during childhood Stops growing during adolescence and then gradually atrophies

Thymus T lymphocyte precursors mature into T lymphocytes Even though the gland atrophies, it continues to produce immunocompetent cells as we age, though at a declining rate

MALT

Mucosa-associated lymphoid tissue (MALT) Lymphoid tissues present in all mucous membranes of the body Function: filter and remove potential pathogens Largest groupings are tonsils, Peyer’s patches in small intestine, and appendix Also present in the mucosa of the respiratory tract (bronchi), genitourinary (GU) organs and the rest of the GI tract

Tonsils

Tonsils Simplest lymphoid organs Form a ring of lymphatic tissue around the pharynx Palatine tonsils Located on either side at the posterior end of the oral cavity. Largest of the three types Lingual tonsils Located at the base of the tongue. Look lumpy. Pharyngeal tonsil Posterior wall of the nasopharynx Aka adenoids

Tonsils Function Gather and remove pathogens entering the pharynx in food or inhaled air

Pharyngeal tonsil Palatine tonsil Lingual tonsil Tonsil Tonsillar crypt Germinal centers in lymphoid follicles Figure 20.8

Peyer’s Patches

Peyer’s Patches Peyer’s patches Clusters of lymphoid follicles In the wall of the distal portion of the small intestine Small intestine does most of the filtration and absorption of food, so it is important to have lymphoid tissue present to help filter out the pathogens/undesirable materials unwanted in the bloodstream Destroy bacteria, preventing them from breaching the intestinal wall Generate “memory” lymphocytes for long term immunity

Lymphoid nodules (follicles) of Peyer’s patch Smooth muscle in the intestinal wall Figure 20.9

Appendix

Appendix Appendix Tubular offshoot of the first part of the large intestine Contains high concentration of lymphoid follicles Destroy bacteria, preventing them from breaching the intestinal wall Generate “memory” lymphocytes for long term immunity

REVIEW

collect excess fluid from interstitial spaces. Lymphatic vessels: collect excess fluid from interstitial spaces. drain blood from lymph nodes. are part of the venous system. are built like arteries. Answer: a. collect excess fluid from interstitial spaces.

Lymphatic capillaries owe their permeability to the presence of: an open end that siphons fluid from the interstitial fluid. minivalves in their walls. tight junctions between adjacent cells. proteins in the lymph fluid. Answer: b. minivalves in their walls.

Surgery always results in tissue swelling. Why would removal of lymph vessels from the right axillary region cause edema in the right arm? Excess blood plasma builds up in regions where there are no lymph vessels. Surgery always results in tissue swelling. Blood is inefficiently circulated in areas where surgery has occurred. Both b and c are true. Answer: a. Excess blood plasma builds up in regions where there are no lymph vessels.

Blood capillaries, arteries, lymphatic vessels, subclavian vein Which of the following represents a logical sequence of structures that contribute to the return of fluid into the blood via the lymphatic system? Blood capillaries, arteries, lymphatic vessels, subclavian vein Arteries, lymphatic vessels, blood capillaries, subclavian vein Arteries, blood capillaries, lymphatic vessels, subclavian vein Subclavian vein, arteries, blood capillaries, lymphatic vessels Answer: c. Arteries, blood capillaries, lymphatic vessels, subclavian vein

The pumping of the heart Which of the following contribute(s) to the flow of lymph through lymphatic vessels? The pumping of the heart The milking action of skeletal muscle contraction Pressure changes in the thorax that result from breathing Both b and c Answer: d. Both b and c

Which of the following would the body perceive as an antigen? Bacteria Virus Mismatched red blood cell All of the above Answer: d. All of the above

all of the above take place. An advantage to having reticular connective tissue present in lymph nodes would be that: the loose organization of reticular connective tissue provides niches for macrophages and lymphocytes. lymph filters through the nodes and is exposed to lymphocytes and macrophages. lymphocytes cycle to various areas of the body and back to lymph nodes. all of the above take place. Answer: d. all of the above take place.

The highest density of lymph nodes can be found ________. in the arms and legs in the brain at the locations where lymphatic collecting vessels converge in the intestines Answer: c. at the locations where lymphatic collecting vessels converge

Lymph tends to stall inside lymph nodes. This is due to: small-diameter spaces within the medulla. the large volume of lymph entering the node. fewer efferent lymphatic vessels than afferent lymphatic vessels. fewer afferent lymphatic vessels than efferent lymphatic vessels. Answer: c. fewer efferent lymphatic vessels than afferent lymphatic vessels.

_________ can enter lymph capillaries. Bacteria Large proteins Interstitial fluid All of the above Answer: d. All of the above

The ______ is the largest lymphoid organ. lymph node Spleen Thymus tonsil Answer: b. spleen

immunity; new red blood cell formation Red pulp of the spleen is most concerned with _______, while white pulp is most concerned with _________. immunity; new red blood cell formation filtering lymph; T lymphocyte formation blood filtering; immunity urine formation; blood filtering Answer: c. blood filtering; immunity

Which of the following lymphoid tissues removes defective red blood cells from the blood? Lymph node Thymus Spleen Peyer’s patches Answer: c. Spleen

The thymus is important for: T lymphocyte maturation. removal of foreign antigens. B lymphocyte maturation. secretion of hormones that promote B lymphocyte immunocompetence. Answer: a. T lymphocyte maturation.

Tonsils promote memory of pathogens by: secreting antibodies into the blood. trapping food particles to support bacterial growth. trapping pathogens to develop immune cells with memory. producing B lymphocytes. Answer: c. trapping pathogens to develop immune cells with memory.