Chapter 20: Lymphatic System

OVERVIEW Two most important functions: maintain fluid balance in the internal environment and immunity Third function is to collect absorbed fat from the intestines and transport it to the systemic veins Lymphatic system: component of the circulatory system; composed of lymph, lymphatic vessels, and isolated structures containing lymphoid tissue: lymph nodes, aggregated lymphoid nodules, tonsils, thymus, spleen, and bone marrow

LYMPH AND INTERSTITIAL FLUID Lymph (lymphatic fluid) Clear, watery fluid found in the lymphatic vessels Closely resembles blood plasma in composition but has a lower percentage of protein Interstitial fluid Complex, organized fluid that fills the spaces between the cells and is part of the extracellular matrix Resembles blood plasma in composition with a lower percentage of protein

LYMPHATIC VESSELS (cont.) Structure of lymphatic vessels Similar to veins except lymphatic vessels have thinner walls, have more valves, and contain lymph nodes Functions of the lymphatic vessels Remove high-molecular-weight substances and particulate matter from interstitial spaces

CIRCULATION OF LYMPH From lymphatic capillaries, lymph flows through progressively larger lymphatic vessels to reenter blood at the junction of the internal jugular and subclavian veins

LYMPH NODES Structure of lymph nodes Lymph nodes are oval-shaped structures enclosed by a fibrous capsule (Figure 20-9) Nodes are a type of biological filter Once lymph enters a node, it moves slowly through sinuses to drain into the efferent exit vessel

LYMPH NODES (cont.) Locations of lymph nodes Most lymph nodes occur in groups Groups with greatest clinical importance are preauricular lymph nodes; submental and submaxillary groups; and superficial cervical, superficial cubital, axillary, iliac, and inguinal lymph nodes

LYMPH NODES (cont.) Lymph nodes perform two distinct functions Defense Filtration Mechanical filtration: physically stopping particles from progressing further in the body Biological filtration: biologic activity of cells destroys and removes particles Hematopoiesis: lymphoid tissue is the site for the 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 Distribution of lymphatics in the breast Drained by two sets of lymphatic vessels Lymphatics that drain the skin over the breast with the exception of the areola and nipple Lymphatics that drain the underlying substance of the breast, as well as the skin of the areola and nipple

LYMPHATIC DRAINAGE OF THE BREAST (cont.) Lymph nodes associated with the breast More than 85% of the lymph from the breast enters the lymph nodes of the axillary region Remainder of lymph enters lymph nodes along the lateral edges of the sternum

TONSILS Form a broken ring under the mucous membranes in the mouth and back of the throat—the pharyngeal lymphoid ring Palatine tonsils: located on each side of the throat Pharyngeal tonsils: located near the posterior opening of the nasal cavity Lingual tonsils: located near the base of the tongue Tubal tonsils: located near the openings of the auditory (eustachian) tubes Protect against bacteria that may invade tissues around the openings between the nasal and oral cavities; bacteria are trapped in tonsillar crypts and put in close contact with immune system cells

THYMUS Function of the thymus Location and appearance of the thymus Primary central organ of lymphatic system Thymus is pinkish gray in childhood; with advancing age, becomes yellowish as lymphoid tissue is replaced by fat 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 Function of the thymus Plays vital role in immunity mechanism Source of lymphocytes before birth Shortly after birth, secretes thymosin and other regulators, which enables lymphocytes to develop into T cells

SPLEEN Location: in the left hypochondrium, directly below the diaphragm, above the left kidney and descending colon and behind the fundus of the stomach Structure of the spleen Ovoid in shape White pulp: dense masses of developing lymphocytes Red pulp: near outer regions, composed of cords of white blood cells and related cells surrounded by sinusoids

SPLEEN (cont.) Functions of the spleen Defense: remove microorganisms from the blood and phagocytose them Hematopoiesis: monocytes and lymphocytes complete their development in the spleen Red blood cell and platelet destruction: macrophages remove worn-out red blood cells and imperfect platelets and destroy them by phagocytosis; also salvage iron and globin from destroyed red blood cells Blood reservoir: pulp of spleen and its sinuses store 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

THE BIG PICTURE: THE LYMPHATIC SYSTEM AND THE WHOLE BODY Lymphatic system drains away excess water from large areas Lymph is conducted through lymphatic vessels to nodes, where contaminants are removed Lymphatic system benefits the whole body by maintaining fluid balance and freedom from disease