The Lymphatic System

Transcription

1 الكلية االسالمية الجامعة قسم تقنيات التحليالت المرضية االسم التدريسي: ا.د.عبد الهادي صالل المرحلة الثانية المادة : التشريح / النظري م. م. عباس حسين عبيد The Lymphatic System The lymphatic system is a vast collection of cells and biochemicals that travel in lymphatic vessels, and the organs and glands that produce them. The lymphatic system includes a network of vessels that assist in circulating body fluids. The lymphatic system consists of the following: (1) lymph, the fluid the system collects from the interstitial spaces of the tissues and returns to the bloodstream. colorless fluid, similar to blood plasma but low in protein. (2) lymphatic vessels, which transport the lymph. (3) lymphatic tissue, composed of aggregates of lymphocytes and macrophages that populate many organs of the body. (4) lymphatic organs, in which these cells are especially concentrated and which are set off from surrounding organs by connective tissue capsules. Diffuse lymphatic system The lymphatic capillaries extend into interstitial spaces, forming complex networks that parallel those of blood capillaries. Lymph converge to form collecting vessels. Eventually, the collecting vessels converge to form larger lymphatic trunks, each of which drains a major portion of the body. The lymphatic trunks converge to form two collecting ducts, the largest of the lymphatic vessels 1. The right lymphatic duct is formed by the convergence of the right jugular, subclavian, and bronchomediastinal trunks in the right thoracic cavity. It receives 1

2 lymphatic drainage from the right upper limb and right side of the thorax and head, and empties into the right subclavian vein. 2. The thoracic duct, on the left, is larger and longer. It begins just below the diaphragm, anterior to the vertebral column at the level of the second lumbar vertebra. Here, the two lumbar trunks and the intestinal trunk join and form a prominent sac called the cisterna chyli, named for the large amount of chyle that it collects after a meal. As it passes through the thorax, it receives additional lymph from the left bronchomediastinal, left subclavian, and left jugular trunks, then empties into the left subclavian vein. Collectively, this duct therefore drains all of the body below the diaphragm, and the left upper limb and left side of the head, neck, and thorax. Structure of a Lymph Node Lymph nodes vary in size and shape, but are usually less than 2.5 centimeters long and somewhat bean-shaped (figs.1). Blood vessels and nerves join a lymph node through the indented region of the node called the hilum. The lymphatic vessels leading to a node (afferent vessels) enter separately at various points on its convex surface, but the lymphatic vessels leaving the node (efferent vessels) exit from the hilum. A capsule of connective tissue encloses each lymph node and subdivides it into compartments. Masses of B cells and macrophages in the cortex, called lymph nodules (lymph follicles), are the functional units of the lymph node. The spaces within a node, called lymph sinuses, provide a complex network of chambers and channels through which lymph circulates. Lymph nodules occur singly or in groups associated with the mucous membranes of the respiratory and digestive tracts. The tonsils are partially encapsulated lymph nodules. Aggregations of nodules called Peyer s patches are scattered throughout the mucosal lining of the distal portion of the small intestine. 2

3 Figure 1: Structure of lymph node. Locations of Lymph Nodes: Lymph nodes are generally in groups or chains along the paths of the larger lymphatic vessels throughout the body, but are absent in the central nervous system. (Figure 2) shows the locations of the major lymph nodes. Thymus FIG 2: Locations of major lymph nodes. 3

4 The thymus is a soft, bilobed gland enclosed in a connective tissue capsule and located anterior to the aorta and posterior to the upper part of the sternum. The thymus is relatively large during infancy and early childhood, but shrinks after puberty and may be quite small in an adult. In elderly persons, adipose and connective tissues replace lymphatic tissue in the thymus. Connective tissues extend inward from the surface of the thymus, subdividing it into lobules. The lobules house many lymphocytes. Most of these cells (thymocytes) are inactive; however, some mature into T lymphocytes, which leave the thymus and provide immunity. Epithelial cells in the thymus secrete hormones called thymosins, which stimulate maturation of T lymphocytes. Figure : The Thymus. (a) Gross anatomy. (b) Histology. The fibrous capsule of the thymus gives off trabeculae (septa) that penetrate into the gland and divide it into several angular lobules. Each lobule has a dense, dark-staining cortex and a lighter medulla inhabited by T lymphocytes. Reticular epithelial cells and pericytes surround the blood capillaries of the cortex, forming a blood thymus barrier that isolates immature lymphocytes from premature exposure to blood-borne antigens. The medulla exhibits whorls of keratinized cells called thymic corpuscles, which are useful for identifying the thymus histologically. Reticular epithelial cells also secrete several signaling molecules that promote the development and action of T cells, including thymosin, thymulin, thymopoietin, interleukins, and interferon. If the thymus is removed from newborn mammals, they waste away and never develop immunity. Other lymphatic organs also seem to depend on thymosins or T cells and develop poorly in thymectomized animals. The relationship of T cell. 4

5 The Thymus.. Arrangement of the reticular epithelial cells to form the blood thymus barrier Spleen The spleen, the largest lymphatic organ, is in the upper left part of the abdominal cavity, just inferior to the diaphragm and posterior and lateral to the stomach. The spleen resembles a large lymph node and is subdivided into lobules. However, unlike the sinuses of a lymph node, the spaces (venous sinuses) in the spleen are filled with blood instead of lymph. The parenchyma exhibits two types of tissue named for their appearance in fresh specimens (not in stained sections): red pulp, which consists of sinuses gorged with concentrated erythrocytes, and white pulp, which consists of lymphocytes and macrophages aggregated like sleeves along small branches of the splenic artery. In tissue sections, white pulp appears as an ovoid mass of lymphocytes with an arteriole passing through it. However, its three-dimensional shape is not egg like but cylindrical. 5

6 These two tissue types reflect the multiple functions of the spleen. Its blood capillaries are very permeable; they allow RBCs to leave the bloodstream, accumulate in the sinuses of the red pulp, and reenter the bloodstream later. The spleen is an erythrocyte graveyard old, fragile RBCs rupture as they squeeze through the capillary walls into the sinuses. Macrophages phagocytize their remains, just as they dispose of blood-borne bacteria and other cellular debris. The spleen produces erythrocytes in the fetus and may resume this role in adults in the event of extreme anemia. Lymphocytes and macrophages of the white pulp monitor the blood for foreign agents, much like lymph nodes monitor the lymph. The spleen is a reservoir for a large standing army of monocytes, waiting in a state of emergency preparedness. In such events as microbial infection, myocardial infarction, or gaping wounds, the hormone angiotensin II stimulates the spleen to release great numbers of monocytes into the bloodstream. The monocytes help to combat infection and repair damaged tissues. The spleen also helps to stabilize blood volume by transferring excess plasma from the bloodstream into the lymphatic system. 6

7 Tonsils The tonsils are patches of lymphatic tissue located at the entrance to the pharynx, where they guard against ingested and inhaled pathogens. Each is covered by an epithelium and has deep pits called tonsillar crypts lined by lymphatic nodules. The crypts often contain food debris, dead leukocytes, bacteria, and antigenic chemicals. Below the crypts, the tonsils are partially separated from underlying connective tissue by an incomplete fibrous capsule. There are three main sets of tonsils: (1) a single median pharyngeal tonsil (adenoids) on the wall of the pharynx just behind the nasal cavity. (2) a pair of palatine tonsils at the posterior margin of the oral cavity. (3) numerous lingual tonsils, each with a single crypt, concentrated in a patch on each side of the root of the tongue. The palatine tonsils are the largest and most often infected. Tonsillitis is an acute inflammation of the palatine tonsils, usually caused by a Streptococcus infection. Their surgical removal, called tonsillectomy,10 used to be one of the most common surgical procedures performed on children, but it is done less often today. Tonsillitis is now usually treated with antibiotics. 7