7.3 Lymphatic circulatory system

The lymphatic system is composed of lymph vessels, lymph ducts, lymph nodes and organs. The organs associated with the lymphatic system are the spleen and thymus. The spleen is the boundary between the blood and the lymphatic system. Knots of lymphatic tissue in the spleen add lymphocytes to the blood. The spleen also acts as a filter for the blood, and helps to destroy worn out red-blood cells. In the event of damage to the spleen, it can be removed and its functions will be carried out by the liver, the bone marrow and the lymph nodes.

Lymph vessels are located as a network throughout all tissues in the body. Lymph vessels assist the circulatory system and all the cells of the body by removing wastes, germs and excess water from the tissue fluid. Lymph vessels carry lymph fluid from the bottom of the body up towards the heart and also drains from the head and shoulders as well as the arms.

Muscle contractions help push the lymph fluid upwards, and valves prevent the lymph fluid from flowing backwards. Many lymph vessels eventually merge into two large lymphatic vessels, called lymphatic ducts which empty into veins in the neck. The thoracic duct collects from the left side of the body and the lower right side of the body and empties into the left subclavian vein. The right thoracic duct collects from the right arm, thorax, neck and head, and drains into the right subclavian vein.

Most of the disease-fighting function of the adult mammal is carried out by the lymph nodes which occur along the lymph ducts. Lymph nodes are small, irregularly-shaped masses through which lymph vessels flow. Clusters of nodes occur in the armpits, groin, and neck. Cells of the immune system line channels through the nodes and attack bacteria and viruses travelling in the lymph, so they basically act as tiny filters.

The main functions of the lymphatic system are as follows:

• The main function of the lymphatic system is to collect and transport tissue fluids from the intercellular spaces in all the tissues of the body, back to the veins in the blood system.
• Lymph plays an important role in returning plasma proteins to the bloodstream.
• Digested fats are absorbed and then transported from the villi in the small intestine to the bloodstream via the lymph vessels.
• Lymphocytes are manufactured in the lymph nodes
• Antibodies manufactured in the lymph nodes assist the body to build up an effective immunity to infectious diseases.
• Lymph nodes play an important role in the defence mechanism of the body. They filter out micro-organisms (such as bacteria) and foreign substances such as toxins.
• Lymph transports large molecular compounds (such as enzymes and hormones) from their manufactured sites to the bloodstream.