1

1. General Composition and Purpose of the Lymphatic System

Purpose:

• Maintains fluid balance by returning excess interstitial fluid (~3L/day) back to blood.

• Absorbs fats and fat-soluble vitamins from the digestive tract (via specialized lymphatic vessels called lacteals).

• Defends the body by housing immune cells (lymphocytes, macrophages), filtering pathogens, and facilitating immune responses.

a. Lymph

• Clear, slightly yellowish fluid derived from interstitial fluid.

• Contains water, proteins, lymphocytes, macrophages, cellular debris, and sometimes pathogens.

• When interstitial fluid enters lymphatic capillaries, it is called lymph.

• Volume of lymph formed daily is about 3 liters.

• Lymph moves only toward the heart (one-way system).

b. Lymphatic Vessels

• Network begins with tiny lymphatic capillaries which merge into larger vessels.

• Flow sequence:
  Lymphatic capillaries → Collecting lymphatic vessels → Lymph nodes → Lymphatic trunks → Lymphatic ducts

Major Vessels:

• Lymphatic Capillaries:

  • Microscopic, blind-ended tubes.

  • Located in almost every tissue except CNS, cartilage, bone marrow, and cornea.

  • Overlapping endothelial cells act as one-way valves allowing interstitial fluid entry but preventing exit.

  • Anchoring filaments connect capillaries to surrounding connective tissue to prevent collapse.

• Collecting Lymphatic Vessels:

  • Larger vessels that collect lymph from capillaries.

  • Have three tunics like veins: intima, media, adventitia.

  • Contain numerous valves to prevent backflow.

• Lymph Nodes:

  • Small, bean-shaped structures along vessels that filter lymph.

• Lymphatic Trunks:

  • Large vessels that drain lymph from specific body regions.

  • Examples: Jugular trunks, subclavian trunks, bronchomediastinal trunks, lumbar trunks, intestinal trunk.

• Lymphatic Ducts:

  • Right Lymphatic Duct: Drains lymph from right upper limb, right side of head and thorax into right subclavian vein.

  • Thoracic Duct: Drains lymph from rest of body into left subclavian vein; begins as cisterna chyli in abdomen.

c. Lymphatic Collecting Vessel and Capillary Structure

• Capillaries:

  • Thin walls of endothelial cells overlapping loosely.

  • Lack basement membrane.

  • Anchoring filaments pull openings wider when tissue swells.

• Collecting vessels:

  • Similar to veins but thinner walls and more valves.

  • Transport lymph through nodes, filtering pathogens.

  • Flow assisted by skeletal muscle contractions, arterial pulsations, breathing movements.