Structure and Function Lymphatic System Structure and Function

Lymphatic System One of the functions of the lymphatic system is to return excess tissue fluid, from tissues to the blood. In this respect, the lymphatic system is a subsidiary circulatory system.

Formation of Tissue Fluid At the arteriole end of a capillary, blood pressure is high; this forces plasma out of the capillary. When it leaves the capillary, it is then referred to as tissue fluid. Tissue fluid transports essential substances such as nutrients and oxygen to body cells.

Most tissue fluid (90%) returns to the venous end of capillaries, carrying wastes. The remaining 10%, is picked up by lymphatic capillaries to return tissue fluid to the blood. If the excess tissue fluid does not return to blood, tissues swell and become waterlogged. The lymphatic system acts like a drainage system. Approx. 21 litres of TF is formed each day

Lymphatic capillaries are microscopic, blind-ended vessels that arise in body tissues. They are one cell thick, like blood capillaries. Unlike blood capillaries (that allow fluid to move in 2 directions), lymph capillaries only allow tissue fluid in and not out again. This is due to the arrangement of endothelial cells that overlap each other and act like mini valves Once inside, the fluid is referred to as lymph.

Blind-ended lymph capillary permeating body tissue

Blood capillaries are continuous with arterioles and venules Lymphatic capillaries are blind-ended

Lymphatic capillaries join to form larger lymph vessels. Lymph vessels return lymph (to blood) at low pressure. Most lymph vessels travel against gravity, taking lymph towards the heart. To assist the flow of lymph, lymph vessels have valves in their walls to prevent lymph pooling in the lower limbs. Lymphatic vessels also have smooth muscle in their walls. This muscle contracts to push lymph through. Like veins, skeletal muscles press against lymph vessels to push lymph in the right direction. This action is called ‘milking’. Valves ensure one-way flow of lymph. Breathing movements also aid in the flow of lymph. Along the length of lymph vessels are collections of lymphatic tissue called lymph nodes.

Lymph nodes filter lymph. They contain lymphocytes that combat pathogens and cancer cells. Lymph nodes also contain macrophages, which are a type of phagocyte. Lymph nodes are bean-shaped structures and are 2.5 cm in diameter (or smaller).

Afferent vessel takes lymph to the node. Efferent vessel takes lymph away from the node. Tough capsule maintains the node’s shape Trabeculae divide the nodes into compartments Cortex – outer part of node Germinal centres – lymphocytes mature Medulla contains macrophages

Normally, there are more afferent vessels than efferent vessels. This slows down the flow of lymph through the node and allows more time for lymph to be cleaned and filtered. Usually, lymph nodes are arranged in clusters and are named after the regional body term of where they are situated.

Lymph passes through a series of nodes before being returned to the blood. Eventually, lymph vessels merge to form one of two vessels: Right lymphatic duct Thoracic duct

The right lymphatic duct drains lymph into the right subclavian vein. In turn, the right subclavian vein drains blood into the superior vena cava. The right lymphatic duct drains lymph from the right side of the head, right upper trunk and the right arm.

The thoracic duct is a lot larger than the right lymphatic duct (40 cm) It drains into the left subclavian vein, which in turn merges with the superior vena cava. The thoracic duct drains lymph from the legs, left trunk, arm and side of head.

Right lymphatic duct Subclavian veins Thoracic duct

Tonsils are collections of lymphatic tissue arranged at the mouth-pharynx junction for protection against inhaled/ingested pathogens. Tonsils contain lymphocytes and phagocytes. 1 adenoid 1 pair lingual tonsils 1 pair of palatine tonsils

The thymus gland is a paired organ situated above the heart. It is active in childhood and in adulthood atrophies. Its role is to produce hormones and mature a type of lymphocyte called a T-lymphocyte. Thymus Spleen

The spleen is the largest mass of lymphatic tissue in the body. It is different in that it filters blood and not lymph. Lymphocytes in the spleen remove any pathogens. Other functions of the spleen include: Blood reservoir Production of RBCs (foetus) Destroy worn out RBCs

Lacteals are blind ended structures within villi of the SI. The role of lacteals is to absorb fatty acids, glycerol and fat-soluble vitamins. These are transported in lymph to enter the circulatory system at the subclavian veins.

Peyer’s patches are a collection of lymph nodes in the ileum of the SI, similar to tonsils. They contain macrophages that can engulf any pathogens taken in by food, preventing them from penetrating the intestinal wall.

Heart Arteries Arterioles Capillaries Venules Veins Subclavian veins Lymph ducts Lymph nodes Lymphatic vessels 100% Tissue fluid Lymphatic capillaries 90% 10%