Chapter 20 Lymphatic System

Overview of the Lymphatic System Importance of the lymphatic system Two most important functions— maintain fluid balance in the internal environment and promote body’s immunity Lymph vessels act as “drains” to collect excess tissue fluid and return it to the venous blood just before it returns to the heart

Overview of the Lymphatic System specialized component of the circulatory system made up of lymph, lymphatic vessels, and isolated structures containing lymphoid tissue: lymph nodes, aggregated lymphoid nodules, tonsils, thymus, spleen, and bone marrow Transports tissue fluid, proteins, fats, and other substances to the general circulation Lymphatic vessels begin blindly in the intercellular spaces of the soft tissues- do not form a closed circuit

Lymph and Interstitial Fluid Lymph (lymphatic fluid) Clear, watery-appearing fluid found in the lymphatic vessels Closely resembles blood plasma in composition

Lymph and Interstitial Fluid Interstitial fluid (IF) Complex, organized fluid that fills the spaces between the cells Resembles blood plasma in composition Along with blood plasma, constitutes the extracellular fluid

Lymphatic Vessels Distribution of lymphatic vessels (lymphatics) (Figures 20-2 and 20-3) Lymphatic capillaries microscopic blind-end vessels where lymphatic vessels originate wall consists of a single layer of flattened endothelial cells networks branch and reconnect freely Lymphatic capillaries merge to form larger lymphatics drains into subclavian vein

Lymphatic Vessels Structure of lymphatic vessels Similar to veins except lymphatic vessels have thinner walls, more valves, and contain lymph nodes Lymphatic capillary wall is formed by a single layer of thin, flat endothelial cells As the diameter of lymphatic vessels increases from capillary size, the walls become thicker and have three layers Semilunar valves are present every few millimeters in large lymphatics and even more frequently in smaller lymphatics

Lymphatic Vessels Functions of the lymphatic vessels Remove high weight substances If anything blocks lymphatic return, blood protein concentration and blood osmotic pressure  Lacteals absorb fats and other nutrients from the small intestine

Circulation of Lymph Lymph system re-circulates about 3 liters/day! From lymphatic capillaries, lymph flows through progressively larger lymphatic vessels & eventually reenters blood at the subclavian veins Lymphatic pump Lymphokinesis the movement of lymph can be visualized Lymph moves through the system in right direction of lots of valves

Lymph Nodes Structure of lymph nodes Oval structures Nodes are a biological filter Once lymph enters a node, it moves slowly through sinuses to drain into the efferent exit vessel lined with specialized cells capable of phagocytosis

Lymph Nodes Functions of lymph nodes Defense functions Filtration Mechanical filtration— physically stops particles Biological filtration— biological activity of cells destroys and removes particles Phagocytosis— cells remove microorganisms and other injurious particles from lymph and phagocytose them If overwhelmed, lymph nodes can become infected or damaged If infection blocks nodes, fluid accumulates  swollen Hematopoiesis— lymphoid tissue is site for final stages of maturation of some lymphocytes and monocytes

Lymphatic Drainage of the Breast Clinically important because cancer cells and infections can spread along lymphatic pathways to lymph nodes and other organs of the body Lymph nodes associated with the breast >85% lymph from the breast enters lymph nodes of axillary region Remainder enters lymph nodes along lateral edges of sternum

Tonsils Form a broken ring under the mucous membranes in the mouth and back of the throat—the pharyngeal lymphoid ring (Figure 20-17) Palatine tonsils— each side of throat Pharyngeal tonsils— near posterior opening of nasal cavity (adenoids when swollen) Lingual tonsils— near base of the tongue Protect against bacteria that may invade tissues around the openings between the nasal and oral cavities

Thymus Location and appearance of the thymus Primary organ of lymphatic system Single, unpaired organ located in the mediastinum Thymus is pinkish gray in childhood with advancing age, becomes yellowish as lymphoid tissue is replaced by fat

Thymus Structure of the thymus Pyramid-shaped lobes are subdivided into small lobules Each lobule is composed of a dense cellular cortex and an inner, less dense, medulla Medullary tissue can be identified by presence of thymic corpuscles

Thymus Function of the thymus Plays vital role in immunity mechanism Source of blood cells before birth Shortly after birth, thymus secretes thymosin and other regulators, which enables lymphocytes to develop into T cells

Spleen Location of the spleen— left hypochondrium Structure of spleen Ovoid in shape White pulp— dense masses of developing lymphocytes Red pulp

Spleen Functions of the spleen Defense macrophages lining spleen remove microorganisms from blood and phagocytose them Hematopoiesis monocytes and lymphocytes complete their development in the spleen Red blood cell and platelet destruction macrophages remove worn-out RBCs and imperfect platelets and destroy them salvage iron and globin from destroyed RBCs Blood reservoir stores blood FYI- As response to hemorrhage, spleen can “self-transfuse” about 200 mL of blood!

Cycle of Life: Lymphatic System Dramatic changes throughout life Organs with lymphocytes appear before birth and grow until puberty Postpuberty Organs atrophy through late adulthood Shrink in size Become fatty or fibrous Spleen— develops early, remains intact Overall function maintained until late adulthood Later adulthood Deficiency permits risk of infection and cancer Hypersensitivity— likelihood of autoimmune conditions