The Lymphatic System

• The lymphatic system, closely linked to the blood circulatory system, is present in most vertebrates like mammals and frogs.
• It consists of a network of vessels and glands distributed throughout the body, containing lymphatic fluid (lymph) and cellular components like lymphocytes.
• Lymph formation occurs through filtration in blood capillaries, creating tissue fluid from blood plasma that occupies the interstitial space.
• Some tissue fluid returns to the blood via osmosis, while the remainder becomes interstitial fluid, lacking red blood cells and platelets but containing a few white blood cells.
• Interstitial fluid bathes body cells and must return to the blood circulatory system to maintain blood volume/pressure and prevent tissue swelling.
• The lymphatic system collects interstitial fluid through hydrostatic pressure and transports it back to venous blood.
• It serves as an accessory to the blood circulatory system by collecting and transporting filtered tissue fluid from interstitial spaces back to the venous blood.

Purposes of the Lymphatic System

• Drainage system for interstitial/tissue fluid.
• Transports filtered tissue fluid back to the blood.
• Defends the body against infections with immune cells like lymphocytes and macrophages that fight disease-causing microorganisms.
• Absorbs emulsified fats/lipids and vitamins (A, D, E, K) from the intestine.
• Lymph from the intestine, called chyle, appears milky-white due to fat absorption and is transported via lacteals.

Parts of the Lymphatic System

• Lymph
  • Fluid found and transported within the lymphatic system.
    • Colorless, except for the milky-white lymph from the small intestines.
    • Transported through vessels, drained into venous systems, and filtered by lymph nodes.
    • Composed of a liquid part and lymphocytes.
• Lymph nodes
  • Lymphatic tissues/organs varying in size (10-20 micrometers), shape (oval or reinform), and color (black, creamy white, or light brown).
    • Found in groups or singularly, such as cervical, axillary, inguinal, and mesenteric lymph nodes.
  • Categorized as superficial (along veins) and deep (along arteries).
  • Lymph flows through nodes via afferent vessels and drains through an efferent vessel towards the subclavian veins.
  • Covered by a capsule with a hilum for artery entry and vein/efferent vessel exit.
  • Medulla contains lymphocytes, macrophages, and plasma cells for immune function.
  • Act as immune system organs, filtering harmful microbes via lymphocytes (white blood cells) and macrophages.
• Lymphatic Vessels
  • Vessels formed by uniting lymphatic capillaries, containing valves to ensure one-way lymph flow.
  • Lymph flow is aided by breathing and muscle contractions due to the absence of a pumping organ.
  • Vessels unite to form a major lymphatic system, emptying lymph into the subclavian vein for return to the heart.
  • Absent in tissues and organs like bone, cartilage, central nervous system, eyeball, skin's epidermis, and inner ear.
• Lymphatic Capillaries
  • Structures emanating from tissue spaces, collecting fluid and delivering it to lymphatic vessels for return to the bloodstream.
  • Form a network, made when several capillaries join to form lymphatic vessels.
  • Walls are permeable to interstitial fluid and large molecules like proteins.
  • Colorless, closed at the tip, wider than blood capillaries, and have relatively low pressure.
  • Walls consist of thin endothelium with a poorly established basement membrane.
  • Absent in tissues such as the brain, bone marrow, spinal cord, and splenic pulp.

Main Function of the Lymphatic System

• Collects and transports interstitial fluid and proteins back to the blood, maintaining fluid balance.
• Excess interstitial fluid in tissues causes swelling, known as edema.
• Interstitial fluid becomes lymph upon entering lymphatic capillaries.
• Obstruction of lymphatic vessels causes lymph accumulation and swelling, known as lymphedema, due to injury, surgery, inflammation, or parasitic infection.
• Lymph nodes filter lymph, with macrophages engulfing foreign particles and microorganisms.

Lymphatic System and Circulatory System Interaction

• The lymphatic system is essential for cardiovascular system function by maintaining blood volume and pressure.
• Interstitial fluid forms during blood filtration in capillaries; some is reabsorbed, and some remains in tissues.
• This fluid becomes lymph upon entering the lymphatic system.
• Lymph filtration occurs at lymph nodes before returning to the bloodstream, supporting a healthy immune system.
• Lymph nodes contain immune cells like B cells, T cells, and macrophages.