Chapter 24 Outline Functions of the Lymphatic System Lymph and Lymph Vessels Lymphatic Cells Lymphatic Structures Aging and the Lymphatic System Development of the Lymphatic System
Functions 1. Return interstitial fluid back to the bloodstream 2. Transport lipids and lipid-soluble vitamins into the bloodstream 3. Production and maturation of lymphocytes 4. Generate an immune response against antigens in the interstitial fluid
Lymphatic Capillaries Lymphatic capillaries are closed-ended tubes that are found interspersed among most blood capillary beds. They resemble blood capillaries but they have overlapping endothelial cells that act as one-way valves allowing interstitial fluid a one-way entrance into lymphatic capillaries.
Lymphatic Capillaries The gastrointestinal tract contains specialized lymph capillaries called lacteals that collect not only interstitial fluid, but also lipids and lipid-soluble vitamins. The lymph collected from the gastrointestinal system has a milky color due to the lipid absorption and is called chyle.
Lymphatic Vessels Lymphatic capillaries merge to form lymphatic vessels. They resemble venules, in that they have components of all three vascular tunics and possess valves similar to veins. Afferent lymphatic vessels bring lymph to a lymph node. Efferent lymphatic vessels transport filtered lymph away from the lymph node.
Lymphatic Trunks Left and right lymphatic trunks form from merging lymphatic vessels. Each trunk drains lymph from a specific region of the body, as follows: 1. Jugular trunks—head and neck 2. Subclavian trunks—upper limbs, breasts and superficial thoracic wall 3. Bronchiomediastinal trunks—deep thoracic structures 4. Intestinal trunks—most abdominal structures 5. Lumbar trunks—lower limbs, abdominopelvic wall and pelvic organs
Lymphatic Ducts Lymphatic ducts are formed from the fusion of lymphatic trunks. The right lymphatic duct is located deep to the right clavicle and returns lymph at the junction of the right subclavian and internal jugular veins. The right lymphatic duct returns lymph from the right side of the head and neck, right upper limb and the right side of the thorax.
Lymphatic Ducts The thoracic duct is the largest lymphatic vessel. It begins just inferior to the diaphragm as a rounded saclike structure called the cisterna chyli. The thoracic duct collects lymph from most of the body (excluding the right lymphatic duct drainage). The thoracic duct passes through the aortic opening of the diaphragm and returns lymph into the junction between the left subclavian and internal jugular veins.
Lymphatic Cells There are several types of lymphatic cells that are located in the lymphatic and circulatory systems: Macrophages=Monocytes (leukocytes) Nurse cells=Epithelial at Thymus Dendritic cells=Epithelial at L. Nodules Lymphocytes=Most abundant
Types of Lymphocytes The body contains three types of lymphocytes: 1.T-lymphocytes (T-cells) 2.B-lymphocytes (B-cells) 3.Natural killer (NK cells) All three cell types migrate through the lymphatic system and search for the presence of antigens.
T-Lymphocytes Make up about 70–85% of body lymphocytes They express a plasma membrane coreceptor (CD) that can recognize a particular antigen There are several types of T-lymphocytes; two main groups are: –helper T-lymphocytes –cytotoxic T-lymphocytes
Helper T-Lymphocytes Primarily contain the CD4 coreceptor and are referred to as CD4+ cells or T4 cells Many types of T4 cells, each one responds to a different antigen T4 cells initiate and oversee the immune response in two ways: –present the antigen to other lymphocytes –secrete cytokines, which are hormones that activate other lymphatic cells
Cytotoxic T-Lymphocytes Also called CD8+ cells or T8 cells, they contain the CD8 coreceptor Come in direct contact with infected or foreign cells and kill them Act only after activated by a helper T- lymphocyte that presents an antigen to it
B-Lymphocytes Make up about 15–30% of body lymphocytes Contain antigen receptors to only one antigen and produce immunoglobulins or antibodies to that single antigen B-lymphocytes become activated when presented with an antigen from a helper T-lymphocyte
B-Lymphocytes Most of the activated B-lymphocytes become plasma cells that produce and secrete large amounts of antibodies. Plasma cells may be either short-lived (less than a week) or long-lived (months or years). The long-lived B-lymphocytes are called memory B-lymphocytes and confer years or lifetime immunity to certain antigens.
B-Lymphocytes and Their Role in the Immune Response Figure 24.6
NK (Natural Killer) Cells Also called large granular lymphocytes Relatively small percentage of all lymphocytes Tend to express the CD16 receptors Unlike T-cells and B-cells that respond to one antigen, NK cells can kill a wide variety of infected cells and some cancerous cells
Lymphopoiesis Lymphopoiesis is the process of lymphocyte development. The final result of lymphopoiesis is that the lymphocyte becomes immunocompetent, meaning the cell can participate in the immune response. All lymphocytes originate in the red bone marrow but their maturation sites differ.
Lymphatic Nodules Oval clusters of lymphatic cells with some extracellular matrix but not surrounded by a connective tissue capsule Center of nodule is called the germinal center; contains proliferating B-lymphocytes and macrophages T-lymphocytes located outside the germinal center Lymphatic nodules filter and attack antigens
MALT (Mucosa-Associated Lymphatic Tissue) Lymphatic nodules located in the mucosa of the gastrointestinal, respiratory, genital, and urinary tracts These nodules monitor and respond to antigens that may enter these tracts MALT is very prominent in the ileum; these nodules are called Peyer patches
Tonsils Located mainly in the pharynx Large clusters of lymphatic cells and extracellular matrix that do not have a completed surrounding capsule Outer edges are invaginated to form crypts, which allow for trapping of antigens to be presented to the lymphocytes
Tonsils There are three types of tonsils: 1.Pharyngeal tonsils (adenoids)—located in the posterosuperior wall of the nasopharynx 2.Palatine tonsils—located in the posterolateral wall of the oral cavity 3.Lingual tonsils—located along the posterior one-third of the tongue
Lymphatic Organs Consists of lymphatic cells and extracellular matrix and is completely surrounded by a connective tissue capsule The main lymphatic organs are: –thymus –lymph nodes –spleen
Thymus Bilobed organ located superficial to the heart Consists of two fused thymic lobes, which are divided into lobules Each lobule has an outer cortex and an inner medulla Continues to grow until puberty and then begins to regress in size and function and, in adults, it becomes replaced mostly by adipose connective tissue
Function of Thymus Site of T-lymphocyte differentiation and maturation Cortex contains immature T-lymphocytes Medulla contains mature T-lymphocytes In adulthood, T-lymphocytes can only be produced by cell division and not by the maturation of new cells in the thymus
Lymph Nodes Small, round or oval structures located along the pathway of lymph vessels Typically found in clusters ranging from 1–25 mm in diameter The primary function of a lymph node is to filter antigens from the lymph and initiate an immune response
Structure of Lymph Nodes Surrounded by a tough connective tissue capsule Internal extensions of the capsule, trabeculae, project into the node Lymphatic cells surround the trabeculae and lymphatic sinuses provide a pathway for lymph flow
Structure of Lymph Nodes Lymph node is divided into outer cortex and inner medulla Cortex consists of nodules and sinuses called cortical sinuses The medulla contains medullary cords and medullary sinuses Afferent vessels deliver lymph to the node Lymph exits nodes via efferent vessels at an indentation of the node called the hilum
Spleen There are several functions of the spleen: 1.Initiates an immune response when antigens are found in blood (white pulp function) 2.Serves as a reservoir for erythrocytes and platelets (red pulp function) 3.Phagocytizes old, defective erythrocytes and platelets (red pulp function) 4.Phagocytizes bacteria and other foreign materials
Spleen Largest lymphatic organ in body just lateral to left kidney A splenic artery/vein enter/leave the spleen via a hilum or indentation on its medial surface Spleen surrounded by a dense irregular connective tissue capsule, which sends extensions called trabeculae into the organ
Spleen Trabecular vessels (branches of splenic arteries and veins) extend within the trabeculae. Cells around the trabeculae are subdivided into white pulp and red pulp. Red pulp surrounds each cluster of white pulp.
Red and White Pulp of Spleen The white pulp is associated with the arterial supply and consists of T- and B-lymphocytes and macrophages. In the center of each cluster is a central artery. The red pulp is associated with the venous supply. Red pulp consists of splenic cords and splenic sinusoids that contain erythrocytes, platelets, macrophages, and some plasma cells. Blood cells can easily enter and leave the blood stream in the spleen because of the discontinuous basal lamina of the capillaries in the splenic sinusoids.