Lymphatic System

1. Lymph, lymph vessels and lymph tissues. The lymphatic system acts in helping the immune system defend the body against infectious agents. Organs such as the tonsils, the nodes in the neck or the spleen are components of lymphatic system. When the tonsils become inflamed, the nodes in the neck also are swollen, and the spleen become enlarged, these are signs that these organs are actively engaged in defending the body against potentially harmful substances. The lymphatic system consists of: • a fluid called lymph • vessels called lymphatic vessels that transport the lymph and that penetrate nearly every tissue of the body, • a number of structures and organs containing lymphatic tissue composed of aggregates of lymphocytes and macrophages. • the red bone marrow The lymphatic system, therefore consists of a network of vessels and a collection of tissues and organs that produce immune cells. It has three functions: 1. Fluid recovery. Fluid continually filters from blood capillaries into the tissue spaces. The capillaries reabsorb about 85% of it, but the 15% that they do not absorb would amount, over the course of a day, to 2 to 4 L of water and one-quarter to one-half of the plasma protein. One would die of circulatory failure within hours if this water and protein were not returned to the bloodstream. One task of the lymphatic system is to reabsorb this excess and return it to the blood. Thereafter, once the interstitial fluid passes into lymphatic vessels, it is called lymph. The major difference between interstitial fluid and lymph is location: Interstitial fluid is found between cells, and lymph is located within lymphatic vessels and lymphatic tissue. 2. Immunity. As the lymphatic system recovers tissue fluid, it also picks up foreign cells and chemicalsfrom the tissues. On its way back to the bloodstream, the fluid passes through lymph nodes, where immune cells stand guard against foreign matter. When they detect anything potentially harmful, they activate a protective immune response. Chapter 2: The Lymphatic System. | MD Vazquez-Carretero. Dept. of Physiology. University of Seville 3 3. Lipid absorption. Lymphatic vessels transport lipids and lipid-soluble vitamins (A, D, E, and K) absorbed by the gastrointestinal tract. In the small intestine, specialized lymphatic capillaries called lacteals carry dietary lipids into lymphatic vessels and ultimately into the blood. Lymph Lymph is usually a clear, colorless fluid, similar to blood plasma but low in protein. It originates as tissue fluid that has been taken up by the lymphatic vessels. Its composition varies substantially from place to place. After a meal, for example, lymph draining from the small intestine has a milky appearance because of its lipid content. Lymph leaving the lymph nodes contains a large number of lymphocytes. Lymph may also contain macrophages, hormones, bacteria, viruses, cellular debris, or even traveling cancer cells. Lymphatic Vessels and lymphatic capillaries Lymph flows through a system of lymphatic vessels similar to blood vessels. The thinnest lymph vessels are the lymphatic capillaries which are located in the spaces between cells and unite to form larger lymphatic vessels, which resemble small veins in structure but have thinner walls and more valves. They are closely associated with blood capillaries, but unlike them, they are closed at one end. The lymphatic capillary resembles the anatomic structure of a blood capillary in that its wall is composed of an endothelium. However, lymphatic capillaries are typically larger in diameter than blood capillaries, lack a basement membrane, and have overlapping endothelial cells. These overlapping endothelial cells act as one-way flaps to allow fluid to enter the lymphatic capillary, but prevent its loss. Anchoring filaments help hold these endothelial cells to the nearby structures. The lymphatic capillaries penetrate nearly every tissue of the body, but are absent from the central nervous system, cartilage, cornea, bone, some portions of the spleen and bone marrow. Unlike the endothelial cells of blood capillaries, lymphatic endothelial cells are not joined by tight junctions; indeed, the gaps between them are so large that bacteria, lymphocytes, and other cells and particles can enter along with the tissue fluid. So Chapter 2: The Lymphatic System. | MD Vazquez-Carretero. Dept. of Physiology. University of Seville 4 lymphatic capillaries have greater permeability than blood capillaries and thus can absorb large molecules such as proteins and lipids. Thus, the composition of lymph arriving at a lymph node is like a report on the state of the upstream tissues. At intervals along the lymphatic vessels, lymph flows through lymph nodes which are encapsulated in connective tissue and which are composed of a mass of lymphocytes. So the flow of the lymph is: Interstitial spaces→ lymphatic capillaries→ lymph vessels→ lymph nodes Lymph Trunks and Ducts As lymphatic vessels exit lymph nodes in a particular region of the body, they unite to form lymph trunks. Then, lymph passes from lymph trunks into two main channels: the thoracic duct and the right lymphatic duct, and then drains into venous blood. The right lymphatic duct is smaller tan the thoracic duct and receives lymph from the upper right side of the body. From the right lymphatic duct, lymph drains into venous blood at the junction of the right internal jugular and right subclavian veins. The thoracic duct (left lymphatic duct) is the main duct for the return of lymph to blood and receives lymph from the left side of the head, neck, and chest, the left upper limb, and the entire body inferior to the ribs. The thoracic duct in turn drains lymph into venous blood at the junction of the left internal jugular and left subclavian veins. The circulation of lymph and the forces that contribute its flow will be seen in the chapter “Cardiovascular System and Hemodynamics”. Chapter 2: The Lymphatic System. | MD Vazquez-Carretero. Dept. of Physiology. University of Seville 5 2. Primary lymphatic structures The widely distributed lymphatic organs and tissues are classified into two groups based on their functions: Primary lymphatic organs are the sites where stem cells divide and become immunocompetent that is, capable of give an immune response. The primary lymphatic organs are: • red bone marrow (in flat bones and the epiphyses of long bones of adults) • thymus Red Bone Marrow: As we said in the last chapter the red bone marrow is responsible for hemopoiesis. Among all the formed elements, the two major types of lymphocytes are T- lymphocytes (also called T-cells) and B-lymphocytes (also called B-cells). Most formed elements move from the red bone marrow into the blood following hemopoiesis. However, Tlymphocytes must migrate to the thymus to complete their maturation. The “T” in the name “T- lymphocytes” originates from the requirement of these cells to complete their maturation in the thymus. The role of the T-cells and B-cells will be described in detail in the next chapter. Thymus The thymus is a bilobed organ that is located above the heart and functions in Tlymphocytes maturation. In infants and young children, the thymus is quite large. The thymus continues to grow until puberty when it reaches its maximun size. Thereafter, much of the thymic tissue is replaced by adipose connective tissue. The thymus in a child, consists of two fused thymic lobes, each surrounded by a connective tissue capsule. Fibrous extensions of the capsule, called trabeculae, subdivide the thymic lobes into lobules. Each lobule is arranged into an outer cortex and inner medulla. Both parts are composed primarily of epithelial tissue infiltrated with T-lymphocytes in varying stages of maturation. The cortex contains immature T-lymphocytes (pre-T- lymphocytes) and the medulla contains mature T-lymphocytes. The epithelial cells secrete thymic hormones (e.g., thymulin) that participate in the maturation of T-lymphocytes. Chapter 2: The Lymphatic System. | MD Vazquez-Carretero. Dept. of Physiology. University of Seville 6 3. Secondary lymphatic structures The secondary lymphatic organs and tissues are the sites where most immune responses occur. They include: • lymph nodes, • spleen, • lymphatic nodules (follicles) The thymus, lymph nodes, and spleen are considered organs because each is surrounded by a connective tissue capsule; lymphatic nodules, in contrast, are not considered organs because they lack completely or parcially the capsule. Lymph nodes I.Description of lymph nodes: • Morphology: round or oval encapsulated structures. • Size: from 1 to 25 millimeters • Number: between 500 and 700 throughout the entire body • Location: superficially and deep within the body and along the pathways of lymph vessels; they typically occur in clusters but also are found individually. • Function: filtering of lymph and removal of unwanted substances, and is a site where occurs B- and T- cell activation. The macrophages present in the lymph nodes remove about 99% of the impurities before the lymph leaves the node. On its way to the bloodstream, lymph flows through one lymph node after another and thus becomes quite thoroughly cleansed of impurities II. Each cluster of lymph nodes receive lymph from selected body regions. For example: • the cervical lymph nodes receive lymph from the head and neck • the axillary lymph nodes in the armpit receive lymph from the breast, axilla, and upper limb • the inguinal lymph nodes in the groin receive lymph from the lower limb and pelvis Chapter 2: The Lymphatic System. | MD Vazquez-Carretero. Dept. of Physiology. University of Seville 7 III. Structure of a lymph node: • The lymph arrive the lymph node by numerous afferent lymphatic vessels. • There is typically only one efferent lymphatic vessel, which is located in an area called the hilum or hilus. By this efferent vessel lymph is drained from the lymph node. • The capsule of a lymph node is composed of dense connective tissue that encapsulates the node and sends internal extensions into it, these extensions are called trabeculae. The trabeculae subdivides the node into compartments. • The connective tissue provides a pathway through which blood vessels and nerves may enter the lymph node. • The lymph node regions deep to the capsule are subdivided into an outer cortex and an inner medulla: o The cortex is composed in part of multiple lymphatic nodules. Each lymphatic nodule within the lymph node is divided in two zones: a germinal centre in the centre and a mantle zone in the peripheria. The germinal centre contains both proliferating B-lymphocytes and some macrophages and the mantle zone contains T-lymphocytes, macrophages, and dendritic cells. o The medulla contains strands of connective tissue fibers that support Blymphocytes, T- lymphocytes, and macrophages. o Both the cortex and medulla of a lymph node also contain tiny open channels called lymphatic sinuses (cortical sinuses and medullary sinuses, respectively). IV. Lymph Flow Through Lymph Nodes Lymph enters a lymph node through numerous afferent lymphatic vessels, then it flows through the lymph node sinuses, before typically draining through one efferent lymphatic vessel. Lymph is continuously monitored for the presence of foreign or pathogenic material as it passes through nodes. Chapter 2: The Lymphatic System. | MD Vazquez-Carretero. Dept. of Physiology. University of Seville 8 Macrophages residing in the lymph node remove foreign debris from the lymph. Because the lymph nodes are often found in clusters, after one lymph node receives and filters lymph, the lymph is then transported to another lymph node in the cluster, then to another, and so on. Thus, lymph is repeatedly screened for unwanted substances. Lymphocytes housed within the lymph node also come into contact with foreign substances. An immune response may be initiated after this contact, during which lymphocytes undergo cellular division, especially in the germinal centers. Some of these new lymphocytes remain within the lymph node, whereas others are transported within the lymph and then enter into the blood, to ultimately reach areas of infections (we will see deeper in the next chapter). When a person has an infection, often some lymph nodes are swollen and tender to the touch. This is a condition erroneously termed swollen glands. These enlarged nodes are a sign that lymphocytes are proliferating and attempting to fight an infection. Spleen I. Description of the spleen • Morphology: oval, its superior surface in convex and its inferior surface in concave. • Size: 12 cm in length • Number: 1, is the largest single mass of lymphatic tissue in the body. • Location: in the left hypochondriac region between the stomach and diaphragm. II. Structure of the spleen: • Like lymph nodes, the spleen has a hilum. Through it pass the splenic artery, splenic vein, and lymphatic vessels. • The spleen is surrounded by a connective tissue capsule from which trabeculae extend into the organ. The spleen lacks a cortex and medulla. • The trabeculae subdivide the spleen into regions of white pulp and red pulp. • White pulp consists of spherical clusters of T-lymphocytes, B- lymphocytes, and macrophages, which surround a central artery. Chapter 2: The Lymphatic System. | MD Vazquez-Carretero. Dept. of Physiology. University of Seville 9 • Red pulp, which is the remaining tissue, contains erythrocytes, platelets, macrophages, and B-lymphocytes. III. Function Like the lymph nodes cleaned the lymph of pathogens, the spleen cleans the blood of potentially pathogenic substances. The functions are: • Phagocytosis of bacteria and other foreign materials in the blood as part of the body’s defense. • Phagocytosis of old, defective erythrocytes and platelets from circulating blood. • Blood storage site for both erythrocytes and platelets: the spleen produces blood cells in the fetus and can do it also in adults in the event of extreme anemia or hemorrhage. • Monitoring the blood for foreign antigens, much like the lymph nodes do the lymph. • Stabilization of blood volume by transferring excess plasma from the bloodstream into the lymphatic system. The spleen is highly vascular and vulnerable to trauma and infection. A ruptured spleen can hemorrhage fatally, but is difficult to repair surgically. Therefore a common procedure in such cases is its removal, splenectomy. A person can live without a spleen, but is somewhat more vulnerable to infections. Lymphatic nodules Lymphatic nodules (follicles) are egg-shaped masses of lymphatic tissue that are not surrounded by a connective tissue capsule. Because they are dispersed throughout the lamina propria (connective tissue) of gastrointestinal, urinary, and reproductive tracts and the respiratory airways, lymphatic nodules in these areas are also referred to as mucosa-associated lymphatic tissue (MALT). Although many lymphatic nodules are small and solitary, some occur in multiple large aggregations in specific parts of the body. For example: • Tonsils: located in the pharyngeal region Chapter 2: The Lymphatic System. | MD Vazquez-Carretero. Dept. of Physiology. University of Seville 10 • Peyer´s patches: located in the ileum of the small intestine. • In the appendix. Tonsils: they are strategically positioned to participate in immune responses against inhaled or ingested foreign substances. In accordance with its location there are 3 sets of tonsils: • A single median pharyngeal tonsil (also called adenoid) on the wall of the pharynx just behind the nasal cavity • A pair of palatine tonsils at the posterior margin of the oral cavity: these are the tonsils we can see at the mirror by ourselves: these are the largest and the most often infected. When they suffer an acute inflammation is called tonsilitis. • Numerous lingual tonsils.