The Lymphatic System

• The lymphatic system is a separate circulatory system with three main functions.
  • Returning fluid (lymph) to circulation.
  • Transporting dietary fats.
  • Aiding the immune system.
• The lymphatic system includes:
  • Vessels.
  • Lymph nodes.
• Lymph nodes serve as the "filters of the lymph."
• Primary lymphatic organs:
  • Bone marrow.
  • Thymus.
  • Lymphocytes mature here.
• Secondary lymphatic organs:
  • Lymph nodes.
  • Spleen.
  • Tonsils.
  • Certain tissues.
  • Various mucous membranes.
• The lymphatic system connects to the venous system at the subclavian veins via:
  • Right lymphatic duct (drains the right side of the head, neck, and trunk).
  • Thoracic duct (drains the remainder of the body).
• Lymphatic capillaries:
  • Pick up interstitial fluid lost by the circulatory system and returns it to venous circulation as lymph.
  • Transport dietary fats from the gastrointestinal system; lacteals are lymphatic structures in villi of the small intestine where fats are absorbed through chylomicrons and transported to the venous circulation.
• The lymphatic system contains a large part of the immune system where lymph nodes help destroy pathogens.

Lymphatic Capillaries

• Distributed throughout the interstitium.
• Absent in the central nervous system, bone marrow, and tissues lacking blood flow (epidermis, cartilage).
• Allow one-way fluid flow into the capillary.
• Consist of overlapping simple squamous epithelium forming one-way valves.
• This arrangement increases the ease of permeability and fluid movement toward the venous circulation.
• Form larger structures called lymphatic vessels that are similar to veins with three layers:
  • Inner endothelium.
  • Middle smooth muscle layer.
  • Outer layer of thin fibrous connective tissue.

Lymphatic Vessels

• Contains valves to allow the one way flow of blood.
• Smooth muscle contraction helps in blood movement.
• Some wall cells generate action potentials promoting muscle contraction.
• Lymph fluid moves through skeletal muscle contraction.
  • Pressure is applied externally causing the vessels to constrict.
  • Valves only allow the one way flow.
• Lymph fluid:
  • Moves into vessels in the thoracic cavity because of dilation.
  • Dilation results in decreased thoracic cavity pressure during inspiration.
  • Fluid moves toward the area of lower pressure.
• Asymmetrical lymph drainage:
  • Right: right lymphatic duct drains the right side of the head, neck, and trunk.
  • Left: the thoracic duct drains the remainder of the body.

Lymph Nodes

• Located throughout the lymphatic system, connected by lymphatic vessels.
• Small, oval structures generally not felt unless enlarged or calcified.
• Remove pathogens from lymph.
• Distributed throughout the body, concentrated in cervical, axillary, inguinal, popliteal, and mammary glands.
• Lymph node structure:
  • Dense connective tissue covering.
  • Trabeculated internal structure.
  • Reticular connective tissue forming an interconnected web.
  • Afferent vessels (entry).
  • Efferent vessels (exit).
• Lymph node layers:
  • Outer cortex: contains open areas called sinuses.
  • Inner medulla: contains medullary cords (branching lymphatic tissue) and medullary sinuses.
• White blood cells in lymph nodes:
  • Macrophages in the sinuses phagocytize bacteria and debris.
  • Lymphocytes in the germinal centers activate and move into the bloodstream.

Lymphatic Organs

• Spleen and thymus are lymphatic organs.
• Contain lymphatic tissue (primarily white blood cells like macrophages, lymphocytes, and reticular cells that produce reticular fibers).
• T and B lymphocytes are produced in bone marrow and carried to the lymphatic system.
  • Activation causes them to divide and attach pathogens.
• Lymphatic tissue:
  • Usually in a lymph node or organ.
  • May be in the mucous membranes of the digestive, urinary, respiratory, and reproductive systems (MALT).
• The spleen is located in the left upper quadrant of the abdominal area and is about the size of an adult fist:
  • Structure: Outer connective tissue capsule and trabeculated inner part with red and white pulp; contains venous sinuses.
  • White pulp: lymphatic tissue with arteries.
  • Red pulp: contains white and red blood cells (associated with veins).
• The splenic artery and vein enter/exit at the hilum.
• Blood flows into the spleen :
  • Through the trabeculated network.
  • Cells here can destroy pathogens and trigger the immune system.
  • Also serves as a blood reservoir.