Chapter 23 - Lymphatic System
 
Overview
  The lymphatic system consists of lymphatic vessels, lymph, lymphoid tissues and organs. Lymph originates in the peripheral tissues and is delivered to the venous system.
  Lymph consists of:
  1. Interstitial fluid
  2. Lymphocytes
  3. Macrophages
 
 Functions
  1. Produce, maintain, and distribute lymphocytes.
    Primary lymphoid structures contain stem cells that differentiate into B, T, and NK cells. Primary lymphoid structures include bone marrow and thymus.
    Secondary lymphoid structures are organs or tissue where activated lymphocytes divide to produce additional lymphocytes of the same type. Lymph nodes and tonsils are examples of secondary lymphoid structures.
  2. Maintain normal blood volume and eliminate variation in chemical composition of interstitial fluid.
     Fluid continual leaks from the systemic capillary beds. The net movement of fluid out of the systemic circulation is approximately 72% of blood volume per day. The lymphatic system returns this fluid to the circulation and in the process helps to even out local variation in the composition of interstitial fluid.
   3. Provides an alternative route for the transport of hormones, nutrients and waste.
     Lipids absorbed by the small intestines is absorbed and carried to the circulation by lymphatic vessels called lacteals.
 
Structure of Lymphatic Vessels
     Lymphatic vessels, also called lymphatics, range in size from lymphatic capillaries to large diameter lymphatic ducts.
     Lymphatic capillaries, also known as terminal lymphatics, consists of a complex of blind-ended, small diameter vessels that collect the lymph in the peripheral tissues. Fluid and larger particulate matter, including viruses and bacteria, enter these capillaries between overlapping endothelial cells that act as one-way valves. Interstitial fluid in this way is constantly monitored.
 
Valves of Lymphatic Vessels   
     From the lymphatic capillaries lymph flows into larger and large vessels as they go toward the lymphatic trunks that deliver the blood to the venous system. Pressure within these larger vessels is small and the larger vessels have one-way valves that insure that fluid flows in the right direction. Fluid movement in these vessels is aided by skeletal muscle contraction, respiratory movements that produce pressure gradients and contraction of smooth muscle in the walls of the larger vessels.
     If the flow of lymph in these lymphatic vessels slows or is blocked, the interstitial fluid is not drained from the tissue and the tissue becomes distended and swollen. This condition is called lymphedema.
 
Major Lymph Collecting Vessels
     Lymph from lymph capillaries drain into two types of lymphatic vessels:
     1. Superficial lymphatics that travels with superficial veins in the subcutaneous tissue of skin, lamina propria of mucosae, and areolar connective tissue of serous membranes.
     2. Deep lymphatics that accompany deep arteries and veins and collect lymph from skeletal muscle and viscera of the abdominopelvic and thoracic cavities.
     Lymph from the lymphatic vessels then drains into larger vessels lymphatic trunks. These lymphatic trunks include:
  1. Lumbar trunks
  2. Intestinal trunks
  3. Bronchomediastinal trunks
  4. Subclavian trunks
  5. Jugular trunks
      The lymphatic trunks empty lymph into two lymphatic trunks:
  1. Thoracic Duct
   The thoracic duct collects lymph from both sides of the body inferior to the diaphragm and from the left side superior to the diaphragm. It drains into the left subclavian v. near the junction with the left internal jugular v.
  1. Right Lymphatic Duct
   The right lymphatic duct collects lymph from the right side of the body superior to the diaphragm. It drains into the right subclavian v. near the junction with the right internal jugular v.
 
Lymphocytes
     Go to the lecture notes on blood.
 
Lymphoid Tissue
     Lymphoid tissues are connective tissues dominated by lymphocytes. Lymphoid tissues include:
  Lymphoid nodules
   Lymphoid nodules are dense concentrations of lymphocytes in the areolar connective tissue of the mucous membranes. Nodules are not surrounded by fibrous capsules and often have a pale, central zone, called a germinal center.
  Tonsils
   Tonsils are large concentration of lymphoid nodules in the wall of the pharynx. These tonsils include:
  Pharyngeal tonsil on the posterior superior wall of the pharynx;
  Palatine tonsils on the posterior margin of the oral cavity;
  Lingual tonsils at the base of the tongue.
  Aggregated lymphoid nodules
   Aggregated lymphoid nodules are found in the small intestines and are called Peyer's patches. These are also found in the appendix.
 
Lymphoid Organs
     Lymphoid organs are separated from surrounding tissue by a fibrous capsule. Lymphoid organs include:
  Lymph nodes 
     Lymph nodes vary in size from 1 to 25 mm and are designed to filter and purify lymph before it reaches the venous system.
     Lymph nodes are shaped like lima beans with an indentation called a hilus. The hilus is where blood vessels and nerves enter the node.
     Fibrous extensions from the capsule of the node, called trabeculae,  extend into the node. The capsule and trabeculae provide support for sinuses that allow lymph to flow through the node.
     Lymph is delivered to the lymph node by afferent lymphatics and is carried away from the lymph node by efferent lymphatics.
     The node can be divided into two regions:
  1. Cortex
     The outer region of the node, near the fibrous capsule, is called the cortex. It contains lymphoid nodules with germinal centers.
  2. Medulla
     The medulla is deeper inside the node where elongate masses of cells called medullary cords are found.
   Thymus 
     The thymus is located posterior to the sternum.      
     The thymus reaches is greatest absolute size at puberty and then gradually decreases in size as the functional cells die and are replaced by fibrous connective tissue.
     The thymus is an important primary lymphoid organ where lymphocytes mature into T lymphocytes.
  Spleen
     The spleen is the largest lymphoid organ in the body.
     The functions of the spleen include:
  1. The removal of abnormal rbc's and other cells by phagocytosis.
  2. The storage of iron from recycled from broken down rbc's.
  3. The initiation of the immune response.