Small lymphatic vessels join to form larger tributaries, called lymphatic trunks , which drain large regions. Lymphatic trunks merge until the lymph enters the two lymphatic ducts. The right lymphatic duct drains lymph from the upper right quadrant of the body. The thoracic duct drains all the rest. Like veins , the lymphatic tributaries have thin walls and have valves to prevent backflow of blood. The system also includes all the structures dedicated to the circulation and production of lymphocytes, including the spleen, thymus, and bone marrow.
The lymphatic system plays a prominent role in immune function, fatty acid absorption, and removal of interstitial fluid from tissues. The lymphatic system is the site of many key immune system functions.
It is important to distinguish that immune system functions can happen almost anywhere in the body, while the lymphatic system is its own system where many immune system functions take place. Besides immune system function, the lymphatic system has many functions of its own. It is responsible for the removal and filtration of interstitial fluid from tissues, absorbs and transports fatty acids and fats as chyle from the digestive system, and transports many of the cells involved in immune system function via lymph.
Interstitial fluid accumulates in the tissues, generally as a result of the pressure exerted from capillaries hydrostatic and osmotic pressure or from protein leakage into the tissues which occurs during inflammation.
These conditions force fluid from the capillaries into the tissues. One of the main functions of the lymphatic system is to drain the excess interstitial fluid that accumulates.
The lymphatic system is a blunt-ended linear flow system, in which tissue fluids, cells, and large extracellular molecules, collectively called lymph, are drained into the initial lymphatic capillary vessels that begin at the interstitial spaces of tissues and organs. They are then transported to thicker collecting lymphatics, which are embedded with multiple lymph nodes, and are eventually returned to the blood circulation through the left and right subclavian veins and into the vena cava.
They drain into venous circulation because there is lower blood pressure in veins, which minimizes the impact of lymph cycling on blood pressure. Lymph nodes located at junctions between the lymph vessels also filter the lymph fluid to remove pathogens and other abnormalities. Fluid removal from tissues prevents the development of edema. Edema is any type of tissue swelling from increased flow of interstitial fluid into tissues relative to fluid drainage. While edema is a normal component of the inflammation process, in some cases it can be very harmful.
Cerebral and pulmonary edema are especially problematic, which is why lymph drainage is so important. Abnormal edema can still occur if the drainage components of the lymph vessels are obstructed. Lymphatic vessels are most densely distributed near lymph nodes: bundles of lymphoid tissue that filter the lymph fluid of pathogens and abnormal molecules.
Adaptive immune responses usually develop within lymphatic vessels. Large lymphatic vessels can be broadly characterized into two categories based on lymph node distribution. Lymph nodes are most densely distributed around the pharynx and neck, chest, armpits, groin, and around the intestines.
Afferent and efferent lymph vessels are also most concentrated in these areas so they can filter lymph fluid close to the end of the lymphatic system, where fluid is returned into the cardiovascular system. Conversely, lymph nodes are not found in the areas of the upper central nervous system, where tissue drains into cerebrospinal fluid instead of lymph, though there are some lymph vessels in the meninges.
There are few lymph nodes at the ends of the limbs. The efferent lymph vessels in the left and lower side of the body drain into the left subclavian vein through the thoracic duct, while the efferent lymph vessels of the right side of the body drain into the right subclavian vein through the right lymphatic duct.
The lymphatic vessels start with the collection of lymph fluid from the interstitial fluid. This fluid is mainly water from plasma that leaks into the intersitial space in the tissues due to pressure forces exerted by capillaries hydrostatic pressure or through osmotic forces from proteins osmotic pressure.
When the pressure for interstitial fluid in the interstitial space becomes large enough it leaks into lymph capillaries, which are the site for lymph fluid collection. In comparison to cardiovascular capillaries, lymphatic capillaries are larger, distributed throughout connective tissues, and have a dead end that completely prevents backflow of lymph.
That means the lymphatic system is an open system with linear flow, while the cardiovascular system is a closed system with true circular flow. Lymph flows in one direction toward the heart. Lymph vessels become larger, with better developed smooth muscle and valves to keep lymph moving forward despite the low pressure and adventia to support the lymph vessels.
As the lymph vessels become larger, their function changes from collecting fluid from the tissues to propelling fluid forward. Lymph nodes found closer to the heart filter lymph fluid before it is returned to venous circulation through one of the two lymph ducts.
Lymph circulates to the lymph node via afferent lymphatic vessels and drains into the lymph node in the subcapsular sinus. Lymph transport refers to the transport of lymph fluid from the interstitial space inside the tissues of the body, through the lymph nodes, and into lymph ducts that return the fluid to venous circulation. Lymphatic capillaries are the site of lymph fluid collection from the tissues.
The fluid accumulates in the interstitial space inside tissues after leaking out through the cardiovascular capillaries. The fluid enters the lymphatic capillaries by leaking through the minivalves located in the junctions of the endothelium. Under ordinary conditions these minivalves prevent the lymph from flowing back into the tissues. In addition to interstitial fluid, pathogens, proteins, and tumor cells may also leak into the lymph capillaries and be transported through lymph. The lymph capillaries feed into larger lymph vessels.
The lymph vessels that receive lymph fluid from many capillaries are called collecting vessels. Semilunar valves work together with smooth muscle contractions and skeletal muscle pressure to slowly push the lymph fluid forward while the valves prevent backflow.
The collecting vessels typically transport lymph fluid either into lymph nodes or lymph trunks. Lymph circulates to the lymph node via afferent lymphatic vessels. The lymph fluid drains into the node just beneath the capsule of the node into its various sinus spaces.
These spaces are loosely separated by walls, so lymph fluid flows around them throughout the lymph node. The sinus space is filled with macrophages that engulf foreign particles and pathogens and filter the lymph. The sinuses converge at the hilum of the node, where lymph then leaves the node via an efferent lymphatic vessel toward either a more central lymph node or a lymph duct for drainage into one of the subclavian veins.
The lymph nodes contain a large number of B and T lymphocytes, which are transported throughout the node during many components of the adaptive immune response. When a lymphocyte is presented with an antigen such as by an activated helper T cell , B cells become activated and migrate to the germinal centers of the node, where they proliferate and differentiate to be specific to that antigen. When antibody-producing B cells are formed, they migrate to the medullary central cords of the node.
Stimulation of the lymphocytes by antigens can accelerate the migration process to about ten times normal, resulting in the characteristic swelling of the lymph nodes that is a common symptom of many infections. The lymphocytes are transported through lymph fluid and leave the node through the efferent vessels to travel to other parts of the body to perform adaptive immune response functions. Flow of Lymph : The lymph flows from the afferent vessels into the sinuses of the lymph node, and then out of the node through the efferent vessels.
After leaving the lymph node through efferent vessels, lymph travels either to another node further into the body or to a lymph trunk, the larger vessel where many efferent vessels converge. Four pairs of lymph trunks are distributed laterally around the center of the body, along with an unpaired intestinal trunk. The lymph trunks then converge into the two lymph ducts, the right lymph duct and the thoracic duct.
These ducts take the lymph into the right and left subclavian veins, which flow into the vena cava. This is where lymph fluid reaches the end of its journey from the interstitial space of tissues back into blood circulation. After the fluid enters the lymph capillaries, it is called lymph. The second function of the lymphatic system is the absorption of fats and fat- soluble vitamins from the digestive system and the subsequent transport of these substances to the venous circulation.
The mucosa that lines the small intestine is covered with fingerlike projections called villi. There are blood capillaries and special lymph capillaries, called lacteals , in the center of each villus.
0コメント