Lymphatic System

   Introduction

   As blood circulates, some of its fluid components push out of the capillary bed into the
   surrounding tissue. This material forms lymph, a special protein-containing tissue fluid that
   bathes the cells. Lymphatic vessels reabsorb part of this lymph to return it to the
   circulation, thereby maintaining tissue fluid balance. The lymphatics also engage in
   absorption of fats and other substances from the digestive tract. Lymph node structures along
   the route of the lymphatics filter out foreign materials and disease-causing agents from the
   general circulation. Other lymphatic system structures include the tonsils, spleen, and
   thymus.

   

   

   

   Capillary hydrostatic pressure: fluid diffusion and reabsorption

   Capillary hydrostatic pressure (filtration pressure) forces fluid out of the blood
   capillaries. Hydrostatic pressure results from the heart forcing blood through the narrow
   arterial part of capillaries. The fluid contains oxygen and nutrients that move into the
   surrounding tissue where they are less concentrated. Similarly, the tissue contains carbon
   dioxide and waste products that move into the capillaries where they are less concentrated.
   This process of substances moving from areas of higher concentration to areas of lower
   concentration is diffusion.

   Fluid reabsorption begins in the lymph capillaries that are throughout the body near blood
   capillaries. Lymph capillaries are small microscopic tubes that collect extracellular fluid.
   The walls of lymph capillaries comprise loosely joined cells. The overlapping edges of the
   cells form mini-valves that allow extracellular fluid to pass into the capillary and prevent
   fluid from flowing back into the tissue. Unlike blood capillaries, lymph capillaries are
   blind-end tubes that lead away from the tissue.

   

   

   Lymph vessels

   Lymph travels through the lymph capillaries to small lymph vessels. Like veins, the walls of
   lymph vessels have smooth muscle that contracts and propels lymph away from the tissues. Lymph
   vessels contain valves that prevent lymph from flowing backward.

   The lymph vessels converge into two main collecting ducts: the shorter right lymphatic duct
   and the longer thoracic duct. The right lymphatic duct drains lymph from the right side of the
   head, neck, thorax, and right upper extremity into the right subclavian vein. Lymph from the
   rest of the body flows into the thoracic duct that empties into the left subclavian vein. The
   thoracic duct begins in the abdomen as an expanded sac called the cisterna chyli. When lymph
   empties into the veins, it forms plasma (the liquid part of blood).

   

   Lymph organs: nodes, nodules, spleen, thymus gland, tonsils

   The lymphoid organs are the lymph nodes, spleen, thymus, and groups of lymph nodules in both
   the oral cavity (tonsils) and small intestine, and appendix (Peyer's patches). A connective
   tissue capsule surrounds the lymph nodes. The nodes have an outer cortex and inner medulla.
   Within the medulla is the germinal center that produces lymphocytes. These infection-fighting
   white blood cells produce antibodies that identify and destroy antigens.

   

   

   

   

   Designed like filters, lymph nodes remove antigens (foreign bodies) from lymph. Each lymph
   node has several sinuses (inner chambers) that contain lymphocytes. Lymph nodes also contain
   macrophages that help clear the lymph of bacteria, cellular debris, and other foreign
   material. Macrophages attack, ingest (engulf), then kill antigens in a process called
   phagocytosis. Small extensions of the macrophage pull the antigen inside.

   

   Lymph nodules are groups of lymphocytes arranged in round clusters. Many lymph organs contain
   lymph nodules within their substances. Unlike lymph nodes, they cannot filter lymph.

   The spleen is the largest lymphoid organ. It has two types of tissue: the red pulp, which
   contains many red blood cells (erythrocytes) and macrophages; and the white pulp, which stores
   lymphocytes. The macrophages in the red pulp remove foreign substances and damaged or dead
   erythrocytes and platelets from the blood. And, the red pulp stores platelets, which are
   important for blood clotting. The lymphocytes within the white pulp are used for the body
   immune system.

   In the thymus gland lymphocytes become specialized. The thymus plays an important role in
   lymphocyte specialization and immunity.

   The tonsils are paired lymph nodules in the oral cavity. These patches of lymph tissue produce
   lymphocytes. The location of each pair (palatine, pharyngeal, and lingual) determines its
   name. The tonsils protect the throat and respiratory system. Sometimes, the tonsils cannot
   remove all the invading microorganisms and become infected. If the infection is severe and
   chronic, the tonsils may require tonsillectomy (surgical removal).