1. LYMPHATIC SYSTEM Importance of the Lymphatic System Two most important functions – Maintenance of fluid balance in the internal environment. – Immunity Importance of the lymphatic system in maintaining fluid balance. – Plasma filters into the interstitial spaces from blood flowing through the capillaries. – Much of this interstitial fluid is absorbed by tissue cells or reabsorbed by the blood before it flows out of the tissue. A small amount of interstitial fluid is left behind. – If this would continue over even a brief period of time, the increased interstitial fluid would cause massive edema. – This edema would causes tissue destruction or death – Lymphatic vessels that act as "drains" to collect the excess fluid and return it to the venous blood just before it reaches the heart. The lymphatic system is a specialized component of the circulatory system.

2. Homeostasis

3. Lymph

4. Lymphatic System

5. LYMPHATIC SYSTEM The Lymphatic System includes – Regional Lymph nodes located along the paths of collecting vessels. – Isolated nodules of lymphatic patches in the Intestinal Wall (Peyer’s Patches) – Specialized lymphatic organs such as the Tonsils, Thymus, And Spleen. The lymphatic system differs from the circulatory system in that the lymphatics do not form a closed ring or circuit. – Instead, begin blindly in the intercellular spaces of the soft tissues of the body.

6. Lymph Lymph and Interstitial Fluid Definitions: – Lymph--clear, watery fluid found in the lymphatic vessels. – Interstitial fluid--fluid that fills the spaces around cells. In some cases, it is part of the semifluid ground substance. In others, it is bound water in a gelatinous ground substance. Distribution of Lymphatic Vessels: Originate as microscopic blind-end vessels called lymphatic capillaries.

7. Lymph

8. Thoracic duct Originates as a dilated structure the cisterna chyli in the lumbar region of the abdominal cavity. Enters the thoracic cavity via Aortic Opening of the Diaphragm to the right side of thoracic aorta. It ascends upward to root of neck along the border of esophagus then bends downward to terminate in the beginning of Left Brachiocephalic Vein. The main lymph trunk of the body Lymph from the entire body except for the right upper quadrant drains into the thoracic duct which drains into the left internal jugular vein.

9. Structure of Lymphatic Vessels Wall of the lymphatic capillary consists of a single layer of flattened endothelial cells. Lymphatic capillaries merge with other lymphatics to eventually form the main lymphatic trunks. Lymphatic vessels resemble veins in structure with these exceptions: – Lymphatics have thinner walls. – Lymphatics contain more valves. – Lymphatics contain lymph nodes located at certain intervals along their course.

10. Lymph Node Functions of Lymph Nodes Defense functions--filtration and phagocytosis – The structure of the sinus channels within the lymph nodes slows the lymph flow through them. This gives the reticuloendothelial cells that line the channels time to remove microorganisms and other injurious particles (soot) from the lymph and phagocytose them. Sometimes such large numbers of microorganisms enter the node that the phagocytes cannot destroy enough of them to prevent their injuring the node. – An infection of the node, adenitis, then results. Because cancer cells often break away from a malignant tumor and enter the lymphatics, they travel to the node, where they may set up new growths. Hematopoiesis – The lymphatic tissue of lymph nodes serve as the sites of the final stages of maturation for some types of lymphocytes and monocytes that have migrated from the bone marrow. Tonsils Masses of lymphoid tissue located in a protective ring under mucous membranes in the mouth and back of the throat. Help protect against bacteria that may invade tissues in the area around the openings between the oral and nasal cavities. Palatine tonsils--located on each side of the throat. Pharyngeal tonsils--also known as adenoids are near the posterior openings of the nasal cavity. Lingual tonsils--near the base of the tongue

11. LYMPH node

12. Lymph Nodes Lymph nodes are small, flattened, oval or bean shaped organs, which are situated in the course of the collecting lymph vessels. Their size is variable (from a few mm to more than 2 cm). The capsule and trabeculae of lymph nodes are formed by connective tissue. Afferent lymph vessels penetrate the capsule and empty into the subcapsular space. The lymph continues thereafter through cortical and medullary sinuses towards the efferent lymph vessels, which emerge from the hilus of the lymph node. The walls of the sinuses can be traversed freely by all components of the lymph, which allows lymphocytes to enter/leave the lymphoid tissue (as part of their constant circulation) or to get in contact with antigens/antigen-presenting cells that may arrive with the lymph. In lymph nodes we find B- and T-lymphocytes, macrophages and reticular cells.

13. Lymphocytes which are located in the outer cortex of the lymph node are likely to represent B-lymphocytes. They are organised into spherical masses - lymphoid nodules or follicles. Sites within the cortex at which B-lymphocytes have been stimulated to proliferate (by contact with an antigen) appear lighter than the surrounding tissue and allow you to identify the centres of lymphoid nodules. The lighter stained parts of the nodules are called Germinal Centres. Mature B-lymphocytes (plasma cells) are located in cord-like extensions of the lymphoid tissue into the medulla, the medullary cords. T-lymphocytes are located in the more diffuse tissue between the nodules and in the paracortex, i.e. the deep part of the cortex. Macrophages are found Scattered Within The Lymphoid Tissue. In many preparations they are difficult to distinguish from the reticular cells, but if an H&E stain turns out nice, macrophages can be distinguished from the reticular cells in the sinus system of the lymph node. Lymph Nodes

16. Tonsils They belong to MALT but considered organs because they are partially encapsulated Tonsils are covered by epithelium They include: 1.Palatine tonsils 2.Pharyngeal tonsils 3.Lingual tonsils 4.Tubal tonsils Walderye’s ring

17. Palatine Tonsils A pair of them located in the oropahrynx Covered by stratified squamous epthelium 10-20 crypts in each one A Sheet of lymphatic nodules and free lymphocytes below the mucosa A dense capsule separates it from the subjacent tissues

19. Pharyngeal Tonsil One in the oropharynx covered by pseudostratified columnar epithelium Form a thin sheet of lymphoid nodules and diffuse lymphocytes No crypts Very thin capsule Adenoid

21. Lingual Tonsils They are multiple small ones at the base of the tongue Covered by stratified squamous epithelium One crypt for each tonsil

23. Thymus In an unpaired organ consisting of two pyramid-shaped lobes with delicate and finely lobulated surfaces. Is located in the mediastinum, extending up into the neck as far as the lower edge of the thyroid and inferiorly as far as the fourth intercostal cartilage. It is the main lymphoid organ from birth until 12-13 th year. It atrophies later on.

24. Thymus

27. Structure Of The Thymus – The pyramid-shaped lobes of the thymus are subdivided into small lobules by connective tissue septa that extend inward from a fibrous covering capsule. – Each lobule is composed of a sense cellular cortex and an inner less dense medulla. Both cortex and medulla are composed of lymphocytes in an epithelial framework. – c. Medullary tissue can be identified by the presence of rather large laminated spherical structures called thymic corpuscles or Hassall's corpuscles. Functions of the Thymus – Thymus performs two important functions: Serves as a final site of lymphocyte development before birth. – The fetal bone marrow forms immature lymphocytes which then move to the thymus. – Many lymphocytes leave the thymus and circulate to the spleen, lymph nodes, and other lymphatic tissue. Soon after birth the thymus begins secreting a group of hormones collectively called thymosin that enable lymphocytes to develop into mature T-cells.

28. T-Cells Differentiation It is the site of terminal differentiation and selection of T-lymphocytes No receptors on the cell surface of precursor cells They divide by mitosis and presented to self antigens bound to class I and II MHC on the surface of epithelial cells, macrophages and dendritic cells Positive and negative selections Mature T-Cells with receptors leave the thymus

29. Spleen Location of the Spleen – Located in the Left Hypochondrium directly below the diaphragm, above the left kidney and descending 9 th 10 th & 11 th Ribs. Structure of the Spleen Roughly ovoid in shape. Is surrounded by a fibrous capsule with inward extensions that roughly divide the organ into compartments. – Arteries leading into each compartment are surrounded by dense masses (nodules) of developing lymphocytes. Because of its whitish appearance, this tissue is called white pulp. – Near the outer regions of each compartment is tissue called red pulp made up of fine reticular fibers submerged in blood that comes from nearby arteries. – After passing through the reticular meshwork, blood collects in venous sinuses and then returns to the heart through veins. Functions of the Spleen Defense As blood passes through the sinusoids, Reticuloendothelial Cell (macrophages) lining these venous spaces remove microorganisms from the blood and destroy them by phagocytosis. Hematopoiesis Monocytes and lymphocytes complete their development to become activated in the spleen. Before birth, red blood cells are also formed in the spleen. Red blood cell and platelet destruction. Macrophages lining the spleen's sinusoids remove worn-out red blood cells and imperfect platelets from the blood and destroy them by phagocytosis. Blood reservoir Pulp of the spleen and its venous sinuses store considerable blood which can be released during hemorrhage

30. Splenic pulp Spleen composed of a network of reticular tissue with reticular cells, lymphocytes, other blood cells, macrophages and APCs White pulp Red pulp

31. White Pulp Blood supply Periarterial lymphatic sheath (PALS) Lymphoid nodule Marginal zine Penicillar arteries

33. Red Pulp Splenic cords supported by reticular fibers and contain T-Cells, B-Cells, macrophages, plasma cells, and many blood cells. Splenic sinusoids Closed circulation Open circulation