The lymphatic system has three main functions, to maintain fluid balance, to defend the body against disease by producing lymphocytes and to absorb fats from the intestine and transport them to the blood.
Lymphatic vessels originate as tiny hair-like capillaries in the interstitial spaces between cells. The walls of these vessels are thin and more permeable than the walls of blood capillaries. These capillaries join together to form larger lymphatic vessels that are similar in structure to veins, but thinner. Like veins, they have valves to prevent lymph passing backwards into the tissue. Unlike the circulatory system however, lymph is not pumped around the body, it depends on the movement of muscles, breathing and gravity. Lymphatic vessels accompany veins and arteries and are often found superficially.
All of the lymphatic vessels eventually converge into either the thoracic duct or the right lymphatic duct. The thoracic duct is about 45 cm long and forms the largest confluence of lymphatic vessels in the body. It receives lymph from the left side of the body as well as the lower right side. It originates at the cisterna chyli, a small pouch that lies on the surface of L2. From here the thoracic duct ascends the thorax, lying to the left of the vertebral column. It then joins with vessels from the neck and empties into the left subclavian vein at its junction with the left internal jugular vein.
The right lymphatic duct is much shorter than the thoracic duct and may only be 1 cm long. It is formed by the confluence of several lymphatic vessels, which drain the right side of the head and thorax, and the right arm. It drains into the junction of the right subclavian and right internal jugular veins.
Lymph nodes are small bean shaped nodules that appear along the course of the lymphatic vessels. A number of afferent lymphatic vessels enter the node and disperse the lymph into the lumen of the node. The lymph is then collected from the lumen and leaves the node in an efferent lymphatic vessel which leaves the node at the hilum. Lymph nodes are full of lymphocytes and macrophages, which are held in place by a matrix of connective tissue and function to;
For more information on lymphocytes see the cardiovascular system: blood chapter.
Lymph nodes usually appear in groups in certain positions of the body;
The spleen is involved with the circulatory and lymphatic systems. It functions to produce white blood cells, recycle old blood cells, as well as fight infection.
The spleen is a bean shaped organ about 12 cm long and 7 cm wide. It lies high on the posterior abdominal wall on the left side of the abdomen, behind the stomach and above the left kidney. The spleen receives blood from the aorta via the splenic artery, and is drained by the splenic vein. It is a mass of splenic pulp held together by a mesh of connective fibres which enclose it in a capsule. The pulp is made up of many types of cell including lymphocytes and phagocytes.
As blood flows through the spleen, any pathogens within it are attacked by lymphocytes in the splenic pulp. Macrophages in the spleen also remove worn-out red and white blood cells and platelets. This breakdown of haemoglobin produces the pigment bilirubin, which is released into the blood plasma. Bilirubin is removed from the blood by the liver and kidneys and is excreted in the bile and to a lesser extent in the urine.
Lymphatic tissue also occurs in places other than specialised lymphatic structures like the nodes or spleen. Lymphatic tissue can be found in the palatine and pharyngeal tonsils, the thymus gland, the small intestine and the appendix.