The lymphatic system acts as a drainage system for the tissues of the body. It works in tandem with the circulatory system, and provides a means of transporting tissue fluids such as lymph from the tissue spaces into the venous system. At this point it becomes part of the circulation of the blood.
Lymphatic capillary vessels are more permeable than their blood vessel counterparts and deal with waste particles which would be otherwise too big to enter the tiny blood capillaries, helping to prevent fluid retention or oedema. The lymphatic system also plays a significant role in protecting the body and providing immunity from disease and infection, by attacking invading micro-organisms and then releasing chemicals and antibodies to fight invading infections. Several types of white blood cell are made, each with a different method of fighting bacteria.
The lymphatic system also plays a role in the transportation of digested fat as well as draining away excess lactic acid formed from general fatigue when muscles are over exercised.
Lymph itself is a watery straw-coloured liquid which is contained within the lymphatic vessels. Its composition is similar to that of blood plasma, however, lymph has a much lower concentration of plasma proteins, no red blood cells and a lot more waste.
The only cells contained within lymph are lymphocytes, which are a type of white blood cell. They are formed in the lymph nodes, thymus and lymphoid tissue sites found throughout the body, for example, the tonsils and spleen. These lymphocytes are responsible for fighting infections. Lymph is also used to transport substances around the body such as fat molecules, and put them back into the blood stream.
When the blood is distributed to the tissues, some of it gets forced out of the capillaries. When this happens, it begins to bathe the tissue cells and becomes interstitial fluid. Some of this passes back into the capillary walls and returns to the bloodstream, whilst the rest is collected by the lymphatic vessels and becomes lymph. The lymph is then taken through the circulatory pathway and returned to the bloodstream.
The lymphatic system is made up of lymphatic capillaries, vessels, nodes and collecting ducts.
Lymph capillaries are found in nearly all tissues with the exception of the central nervous system, bone marrow and any tissues which do not contain blood vessels. Lymphatic vessels are vein-like structures which begin life as lymphatic capillaries. These are minute tubes which join together to form the vessels.
The capillaries form a network in the tissues and drain fluid and waste away from the tissue spaces. As soon as interstitial fluid enters the lymphatic capillaries it becomes lymph and is gathered into the lymphatic vessels. The walls of the lymphatic capillaries have a single cell layer allowing tissue fluid to enter them. However, they are also permeable to larger molecules.
The lymph vessels themselves have thin, collapsible walls and contain a number of valves which help to keep the lymph flowing in the right direction. Superficial lymphatic vessels follow the course of the veins and drain the skin, whilst the deeper lymphatic vessels run with the arteries and drain the internal structures.
Whilst the vessels run parallel to the veins and arteries, they do not complete the same circular flow. They carry lymph towards the heart and once the lymph has been filtered through various nodes, it drains into the respective lymphatic ducts before being returned to the circulatory system via either the right or left subclavian vein.
The lymph nodes can be found at intervals along the lymphatic vessels, and are covered by a capsule of connective tissue. They are made up of lymphatic tissue and are divided into the outer cortex and inner medulla. There are three major collections of lymph nodes which can be found in the groin, armpits and neck. Afferent vessels enter a lymphatic node and efferent vessels drain lymph from the node.
Each lymph node contains up to five afferent vessels and one or two efferent vessels. The nodes themselves are divided by trabeculae, which help to carry the lymph node’s blood supply. Each lymph node receives lymph from vessels and blood from arterioles and capillaries. The function of the lymphatic node is to act as a filter for micro-organisms, debris or harmful substances. This ensures that when the lymph enters the blood it has been completely cleared.
Lymph nodes occur in chains, and from each chain the efferent vessels combine to form trunks which empty into two main ducts known as the thoracic duct and the right lymphatic duct. The purpose of these ducts is to collect lymph and return it to the blood. The thoracic duct collects lymph from the left side of the head and neck, the left arm, lower limbs and abdomen. This then drains into the left subclavian vein in order to return to the bloodstream. The right lymphatic duct collects lymph from the right side of the head and neck, and the right arm. This drains into the right subclavian vein before returning to the bloodstream.
The lymphatic system compliments the circulatory system by draining excess fluid from the tissues and returning it to the circulatory system. This helps fulfil a number of tasks including maintaining blood volume and blood pressure, and preventing oedema.
Lymphatic drainage is the movement of lymph around the body. This movement is assisted by a number of factors, including the pressure from the skeletal muscles against the lymphatic vessels, the respiratory changes in pressure and the compression of the lymph vessels from the pull of the skin and fascia during movement.
Massage helps speed up the lymphatic circulation, as pressure is always directed toward the nearest lymph node. This will help prevent the ‘pooling’ of lymph in the tissues. A working knowledge of the position of the lymph nodes is important because if too much pressure is applied they may swell and start to produce lymphocytes unnecessarily, which would therefore be detrimental to health.