The lymphatic system is a system of drainage vessels which return fluids (lymph) from between the cells to the heart via the veins. The lymphatic system also slowly delivers fatty substances absorbed by the intestine into the blood. This allows the blood to accommodate lipids which might block the flow of blood if they were taken directly into the blood from the digestive system. Another main function of the lymphatic system is fighting microorganism in the immune response. The smallest vessels of the lymphatic system are closed at one end. These microscopic vessels are the lymphatic capillaries. The lympth capillaries unite to form lymphatic vessels, which contain many valve to assure that the lymph in them flows in one direction. Lymphatic vessels flow into lymph nodes, which are situated mainly in the lumbar region, axillae, neck, and in the serosa of the abdomen. The tonsils are a ring of lymph nodes located in the pharynx. Vessels flowing into a node are called afferent lymphatic vessels; on the other hand, lymphatic vessels that exit lymph nodes (from a depression called the hilus) are called efferent lymphatic vessels.
Lymphatic vessels from the left side of the thorax and head, left arm, and everything below the diaphragm on both side of the body, empty into the largest lymphatic vessel, the thoracic duct, which in turn empties into the left subclavian vein. Lymphatic vessels from the right arm, the right side of the thorax, head, and neck empty into the right lymphatic duct, which empties into the right subclavian vein.