Lymphatic system

==Lymphatic system==

Introduction:

  • The lymphatic system comprises a network of vessels that transport body fluids, the cells and chemicals in those vessels and the organs and glands that produce them.
  • Lymphatic vessels collect and carry away excess fluid from interstitial spaces & special vessels called lacteals transport fats to the circulatory system.
  • The organs of the lymphatic system ==help defend against disease.==

Lymphatic pathway

  • start as lymphatic capillaries that merge to form a larger vessel that empties into the circulatory system.

Lymphatic capillaries

  • are tiny, closed-ended tubes that extend into interstitial spaces.
  • receive tissue fluid (lymph) through their thin walls.
  • lymph capillaries are very permeable & collect tissue fluids and proteins.
  • Lacteals are specialized lymph capillaries in the villi of the small intestine
  • they absorb fat-soluble & products of digestion (vitamins ADEK)

Lymphatic vessels

  • the system of lymph vessels begin as dead-end lymph capillaries found in most tissue spaces

  • the walls of lymphatic vessels are thinner than those of vein but are constructed with same three layers with semilunar valves on the inside

  • no pump for lymph

    • kept moving by the same mechanism that promote the venous return
    • the smooth muscle layer of large lymoh vessels constricts, and one way valve prevent backflow of lymph.
    • lymph vessels in the extremities (leg) are compressed by skeletal muscle that surround them (skeletal muscle pump)

  • larger lymphatic vessel pass through the lymph nodes and merge to form lymphatic trunks.

  • Lymph capillaries collect tissue fluid, which is returned to the blood.

Lymphatic trunks and collecting ducts

  • The lymphatic trunks drain lymph from the body and are named for the regions they drain.
  • These trunks join one of two collecting ducts, either
    • The thoracic duct
    • right lymphatic duct.
  • The thoracic duct drains into the left subclavian vein, while the right lymphatic duct drains into the right subclavian vein.
    • Thoracic duct- receives lymph from the left side of the thorax, the left arm, and the left side of the head and neck

tissue fluid and lymph

  • Tissue fluid becomes lymph once it has entered a lymphatic capillary; lymph formation depends on tissue fluid formation.
    • Lymphatic tissue consists mainly of lymphocytes in a mesh-like framework of connective tissue.
    • most lymphocytes are produced from stem cells in the red bone marrow, then migrate to the lymph nodes and nodules, to the spleen, and to the thymus.
    • In these structures, lymphocytes become activated and proliferate in response to infection
  • The thymus has stem cells that produce a significant portion of the T lymphocytes.
tissue fluid formation
  • Tissue fluid is made up of water and dissolved substances that leave blood capillaries by filtration and diffusion.
  • During filtration, some smaller proteins leak from capillaries into the tissues and are not returned to the bloodstream, thus increasing osmotic pressure within the tissues.
lymph formation and function
  • Rising osmotic pressure in tissues interferes with the return of fluids to the bloodstream.
  • Increasing interstitial pressure forces some of the fluid into lymphatic capillaries.

Lymph movement

  • The hydrostatic pressure of tissue fluid drives the entry of lymph into lymphatic capillaries.
  • The forces that propel lymph through lymphatic vessels are forces that move blood in veins (skeletal muscle contraction, breathing movements, and contraction of smooth muscle in the walls of lymphatic trunks).
  • A condition that interferes with the flow in lymph will result in edema. During surgery, lymphatic vessels or tissues may be removed or disturbed, resulting in edema.

Lymph nodes

  • Lymph nodes, which contain lymphocytes and macrophages, are located along lymphatic pathways.
  • Lymph nodes and nodules are masses of lymphatic tissue and differ in size and location.
    • Nodes are usually larger, 10 to 20 mm in length, and are encapsulated.
    • Nodules range from a fraction of a millimeter to several millimeters in length, they do not have capsules.
  • Lymph nodes are found in groups along the pathways of lymph vessels, and lymph flows through these nodes on its way to the subclavian veins. Lymph enters a node through several afferent lymph vessels and leaves through one or two efferent vessels (Figure below).
  • As lymph passes through a lymph node, bacteria and other foreign materials are phagocytized by fixed (stationary) macrophages. Plasma cells develop from lymphocytes exposed to pathogens in the lymph and produce antibodies. These antibodies will eventually reach the blood and circulate throughout the body.

<<What is the function of the plasma cells in a lymph node?<<

  • remove bacteria & clear debris before returned to blood centres of lymphatic production

Structure of lymph node

  • Lymph nodes are bean-shaped, with blood vessels, nerves, and efferent lymphatic vessels attached to the indented hilum, and with afferent lymphatic vessels entering on the convex surface.
  • Lymph nodes are covered with connective tissue that extends inside the node and divides it into nodules and spaces called sinuses.
  • These contain both lymphocytes and macrophages which clean the lymph as it flows through the node.

Location of lymph nodes

  • The lymph nodes generally occur in chains along the parts of the larger lymphatic vessels.

  • Lymph nodules are also found singly or in groups throughout the mucous membranes of the respiratory, urinary, reproductive and digestive tracts.

    • MALT – mucosa associated
    • lymphoid tissue
    • Peyer’s patches in ileum
    • Tonsils (lymphoid tissue under the mucous membranes of the throat) palatine tonsils, pharyngeal tonsil – adenoid, lingual tonsils

  • Some in appendix

Functions of lymph nodes

  • The macrophages and lymphocytes within lymph nodes filter lymph and remove bacteria and cellular debris before lymph is returned to the blood.
  • Lymph nodes are also centers of lymphocyte production; these cells function in immune surveillance.

Tonsils

  • Multiple groups of large lymphatic nodules
  • Location – mucous membrane of the oral and pharyngeal cavities
  • Palatine tonsils
    • Posterior-lateral walls of the oropharynx
  • Pharyngeal tonsil
    • Posterior wall of nasopharynx
  • Lingual tonsils
    • Base of tongue

Thymus and spleen

  • have similar function to lymph nodes
Thymus
  • The thymus is a soft, bi-lobed organ located behind the sternum; it shrinks in size during the lifetime (large in children, microscopic in the elderly).
  • The thymus is surrounded by a connective tissue capsule that extends inside it and divides it into lobules.
  • Lobules contain lymphocytes, some of which mature into T lymphocytes (T cells) that leave the thymus to provide immunity.
  • The thymus secretes the hormone thymosin, which influences the maturation of T-lymphocytes once they leave the thymus.
spleen
  • The spleen lies in the upper left abdominal cavity and is the body’s largest lymphatic organ.
  • The spleen resembles a large lymph node except that it contains blood instead of lymph.
  • Inside the spleen lies white pulp (containing many lymphocytes) and red pulp (containing red blood cells, macrophages, and lymphocytes).
  • The spleen filters the blood and removes damaged blood cells and bacteria.

Lymphatic circulation

  • Fluid removed from tissues enters lymphatic capillaries as lymph. It flows in one direction due to valves.
  • Vessels form trunks and ducts that drain lymph back into the circulatory system.
  • Lacteals absorb fats from diet and this enters lymph which is called chyle.
  • The spleen filters blood and allows lymphocytes to respond and trap infection.
  • B and T lymphocytes originate from bone marrow, and enter blood. B cells mature in marrow whereas pre-T cells mature in the thymus.
  • B and T cells exit the blood and enter tissues. They also can re-enter circulation. They respond to infectious agents.