It comprises lymphatic capillaries lymphatic vessels, lymphatic nodes and lymphatic ducts.
1. Lymph: Lymph is a mobile connective tissue comprising plasma (liquid) and corpuscles (cells). Plasma of lymph is similar to that of blood has fewer blood proteins, less calcium and phosphorus and high glucose concentration. As regards the corpuscles, leucocytes (white blood corpuscles) are numerous which are mainly lymphocytes. Erythrocytes (red blood corpuscles) and platelets are completely absent in lymph.
2. Lymphatic Capillaries: They lie close to the blood capillaries but differ from them to the extent that they end blindly. Moreover, they have extremely thin walls. They are composed of a single layer of endothelial cells. The lymph capillaries of intestine adsorb the digested fats. They are quite milky in appearance and are, therefore, called the lacteals.
3. Lymphatic Vessels: The lymphatic capillaries unite to form larger lymphatic vessels.
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4. Thoracic Duct: The lymphatic vessels of left side unite to form a thoracic duct. This duct begins at the cisterna chyli, which is a sac-dilation situated in front of the first and second lumber vertebrae. The thoracic duct contains several valves. It discharges its lymph into the left subclavian vein.
5. Right Lymphatic Duct: The lymphatic vessels of the right side of the thorax, head and neck unite to form the right lymphatic duct. It is smaller than the thoracic duct. It discharges it lymph into the right subclavian vein.
6. Lymphatic Nodes: The lymphatic vessels bear lymph nodes. Lymph nodes are abundant in the neck, armpit and groin. They contain phagocytic white blood corpuscles and macrophages which eat harmful microorganisms and foreign particles from the lymph.
Lymph nodes also add lymphocytes and antibodies. Lymph moves from lymphatic capillaries to lymphatic vessels, lymphatic ducts and ultimately opens into the venous system.
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Lymph Movement:
The lymph flows slowly. Lymph that returns from the head and neck into large lymphatic ducts, moves easily due to the force of gravity. However, the lymph that moves from those portions of the body below the entrance of the lymph ducts into the venous system depends upon the action of skeletal muscles, breathing movements, movements of the heart, and abdominal organs. It means contractions of the body muscles and movements of viscera help the movement of the lymph. The valves present in the lymphatic vessels prevent the back flow of the lymph. Movement of villi helps flow of lymph in the lacteals. Blocking of lymph flow causes oedema.
Lymphoid Organs:
The organs which secrete lymph are called lymphoid organs. Besides the lymph nodes, tonsils, thymus gland, Peyer’s patches, liver and spleen are the other lymphoidal organs that secrete lymph. The spleen is the largest mass of lymphatic tissue in the body.