| Lymphatic System | |
|---|---|
| An image displaying the lymphatic system. | |
| Latin | systema lymphoideum |
The lymphatic system is, first of all, a part of the circulatory system, comprising a network of conduits called lymphatic vessels that carry a clear fluid called lymph (from Latin lympha "water"[1]) unidirectionally toward the heart. The lymphatic system as we know it today was first described independently by Olaus Rudbeck and Thomas Bartholin.
Secondly, it plays an important part in the immune system, having a considerable overlap with the lymphoid system. Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated with the digestive system such as the tonsils. The system also includes all the structures dedicated to the circulation and production of lymphocytes, which includes the spleen, thymus, bone marrow and the lymphoid tissue associated with the digestive system.[2]
The blood does not directly come in contact with the parenchymal cells and tissues in the body, but constituents of the blood first exit the microvascular exchange blood vessels to become interstitial fluid, which comes into contact with the parenchymal cells of the body. Lymph is the fluid that is formed when interstitial fluid enters the initial lymphatic vessels of the lymphatic system. The lymph is then moved along the lymphatic vessel network by either intrinsic contractions of the lymphatic passages or by extrinsic compression of the lymphatic vessels via external tissue forces (e.g. the contractions of skeletal muscles). Eventually, the lymph vessels empty into the lymphatic ducts, which drain into one of the two subclavian veins (near the junctions of the subclavian veins with the internal jugular veins).
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The adjective used for the lymph-transporting system is "lymphatic". The adjective used for the tissues where lymphocytes are formed is "lymphoid".
The lymphatic system has multiple interrelated functions:[3]
Lymphoid tissue is a specialized connective tissue - reticular connective, that contains large quantities of lymphocytes.
The study of lymphatic drainage of various organs is important in diagnosis, prognosis, and treatment of cancer. The lymphatic system, because of its physical proximity to many tissues of the body, is responsible for carrying cancerous cells between the various parts of the body in a process called metastasis. The intervening lymph nodes can trap the cancer cells. If they are not successful in destroying the cancer cells the nodes may become sites of secondary tumors.
The lymphatic system can be broadly divided into the conducting system and the lymphoid tissue.
Lymphoid tissue associated with the lymphatic system is concerned with immune functions in defending the body against the infections and spread of tumors. It consists of connective tissue with various types of white blood cells enmeshed in it, most numerous being the lymphocytes.
The lymphoid tissue may be primary, secondary, or tertiary depending upon the stage of lymphocyte development and maturation it is involved in. (The tertiary lymphoid tissue typically contains far fewer lymphocytes, and assumes an immune role only when challenged with antigens that result in inflammation. It achieves this by importing the lymphocytes from blood and lymph.[5])
The central or primary lymphoid organs generate lymphocytes from immature progenitor cells.
The thymus and the bone marrow constitute the primary lymphoid tissues involved in the production and early selection of lymphocytes.
Secondary or peripheral lymphoid organs maintain mature naive lymphocytes and initiate an adaptive immune response. The peripheral lymphoid organs are the sites of lymphocyte activation by antigen. Activation leads to clonal expansion and affinity maturation. Mature lymphocytes recirculate between the blood and the peripheral lymphoid organs until they encounter their specific antigen.
Secondary lymphoid tissue provides the environment for the foreign or altered native molecules (antigens) to interact with the lymphocytes. It is exemplified by the lymph nodes, and the lymphoid follicles in tonsils, Peyer's patches, spleen, adenoids, skin, etc. that are associated with the mucosa-associated lymphoid tissue (MALT).
A lymph node is an organized collection of lymphoid tissue, through which the lymph passes on its way to returning to the blood. Lymph nodes are located at intervals along the lymphatic system. Several afferent lymph vessels bring in lymph, which percolates through the substance of the lymph node, and is drained out by an efferent lymph vessel.
The substance of a lymph node consists of lymphoid follicles in the outer portion called the "cortex", which contains the lymphoid follicles, and an inner portion called "medulla", which is surrounded by the cortex on all sides except for a portion known as the "hilum". The hilum presents as a depression on the surface of the lymph node, which makes the otherwise spherical or ovoid lymph node bean-shaped. The efferent lymph vessel directly emerges from the lymph node here. The arteries and veins supplying the lymph node with blood enter and exit through the hilum.
Lymph follicles are a dense collection of lymphocytes, the number, size and configuration of which change in accordance with the functional state of the lymph node. For example, the follicles expand significantly upon encountering a foreign antigen. The selection of B cells occurs in the germinal center of the lymph nodes.
Lymph nodes are particularly numerous in the mediastinum in the chest, neck, pelvis, axilla (armpit), inguinal (groin) region, and in association with the blood vessels of the intestines.[2]
Tubular vessels transport back lymph to the blood ultimately replacing the volume lost from the blood during the formation of the interstitial fluid. These channels are the lymphatic channels or simply called lymphatics.[6]
Lymph vessels called lacteals are present in the lining of the gastrointestinal tract, predominantly in the small intestine. While most other nutrients absorbed by the small intestine are passed on to the portal venous system to drain via the portal vein into the liver for processing, fats (lipids) are passed on to the lymphatic system to be transported to the blood circulation via the thoracic duct. (There are exceptions, for example medium-chain triglycerides (MCTs) are fatty acid esters of glycerol that passively diffuse from the GI tract to the portal system.) The enriched lymph originating in the lymphatics of the small intestine is called chyle. The nutrients that are released to the circulatory system are processed by the liver, having passed through the systemic circulation.
Lymphedema is the swelling caused by the accumulation of lymph fluid,[7] which may occur if the lymphatic system is damaged or has malformations. It usually affects the limbs, though face, neck and abdomen may also be affected.
Some common causes of swollen lymph nodes include infections, infectious mononucleosis, and cancer, e.g. Hodgkin's and non-Hodgkin lymphoma, and metastasis of cancerous cells via the lymphatic system.
Lymphangiomatosis is a disease involving multiple cysts or lesions formed from lymphatic vessels.
In elephantiasis, infection of the lymphatic vessels cause a thickening of the skin and enlargement of underlying tissues, especially in the legs and genitals. It is most commonly caused by a parasitic disease known as lymphatic filariasis. Lymphangiosarcoma is a malignant soft tissue tumor, whereas lymphangioma is a benign tumor occurring frequently in association with Turner syndrome. Lymphangioleiomyomatosis is a benign tumor of the smooth muscles of the lymphatics that occurs in the lungs.
Lymphatic tissues begin to develop by the end of the fifth week of embryonic development. Lymphatic vessels develop from lymph sacs that arise from developing veins, which are derived from mesoderm.
The first lymph sacs to appear are the paired jugular lymph sacs at the junction of the internal jugular and subclavian veins. From the jugular lymph sacs, lymphatic capillary plexuses spread to the thorax, upper limbs, neck and head. Some of the plexuses enlarge and form lymphatic vessels in their respective regions. Each jugular lymph sac retains at least one connection with its jugular vein, the left one developing into the superior portion of the thoracic duct.
The next lymph sac to appear is the unpaired retroperitoneal lymph sac at the root of the mesentery of the intestine. It develops from the primitive vena cava and mesonephric veins. Capillary plexuses and lymphatic vessels spread from the retroperitoneal lymph sac to the abdominal viscera and diaphragm. The sac establishes connections with the cisterna chyli but loses its connections with neighboring veins.
The last of the lymph sacs, the paired posterior lymph sacs, develop from the iliac veins. The posterior lymph sacs produce capillary plexuses and lymphatic vessels of the abdominal wall, pelvic region, and lower limbs. The posterior lymph sacs join the cisterna chyli and lose their connections with adjacent veins.
With the exception of the anterior part of the sac from which the cisterna chyli develops, all lymph sacs become invaded by mesenchymal cells and are converted into groups of lymph nodes.
The spleen develops from mesenchymal cells between layers of the dorsal mesentery of the stomach. The thymus arises as an outgrowth of the third pharyngeal pouch.
Present research has found cues about a lymphatico-venous communication. In mammals, lymphatico-venous communications other than those at the base of the neck are not easy to demonstrate, but described in some experiments.[8]
The specialists observed that the pulmonary complications following lymphangiography (a test which utilizes X ray technology, along with the injection of a contrast agent, to view lymphatic circulation and lymph nodes for diagnostic purposes) are more often severe in patients with lymphatic obstruction. In these cases, the contrast medium is thought to reach the vascular system via lymphovenous communications with shunt the material directly into the venous stream, bypassing those lymph nodes distal to the communications,[9] Because less contrast agent is absorbed in lymph nodes, a greater portion of the injected volume passes into the vascular system. Since pulmonary complications are related to the amount of medium reaching the lungs area, the early recognition of lymphovenous communications is a great significance to the lymphangiographer.[9] Another "hint" in proving a lymph-vein communication is offered by a Robert F Dunn experiment. The passage of radioactively tagged tracers, injected at elevated pressure, through the lymph node-venous communications coincides with the increased pressures of injection and subsequent nodal palpation in dogs. The passage of iodinated I 125 serum albumen (ISA) indicates that direct lymph node-venous communications are present, whereas passage of nucleated erythrocytes requires a communication structure the size of a capillary or larger.[10] Moreover, the evidence suggest that in mammals under normal conditions, mostly of the lymph is returned to the blood stream through the lymphatico-venous communications at the base of the neck. When the thoracic duct-venous communication is blocked, however, the resultant raised intralymphatic pressure will usually cause other normal non-functioning communications to open and thereby allow the return of lymph to the blood stream.[8]
Hippocrates was one of the first persons to mention the lymphatic system in 5th century BC. In his work "On Joints," he briefly mentioned the lymph nodes in one sentence. Rufus of Ephesus, a Roman physician, identified the axillary, inguinal and mesenteric lymph nodes as well as the thymus during the 1st to 2nd century AD.[11] The first mention of lymphatic vessels was in 3rd century BC by Herophilos, a Greek anatomist living in Alexandria, who incorrectly concluded that the "absorptive veins of the lymphatics", by which he meant the lacteals (lymph vessels of the intestines), drained into the hepatic portal veins, and thus into the liver.[11] Findings of Ruphus and Herophilos findings were further propagated by the Greek physician Galen, who described the lacteals and mesenteric lymph nodes which he observed in his dissection of apes and pigs in the 2nd century AD.[11][12]
Until the 17th century, ideas of Galen were most prevalent. Accordingly, it was believed that the blood was produced by the liver from chyle contaminated with ailments by the intestine and stomach, to which various spirits were added by other organs, and that this blood was consumed by all the organs of the body. This theory required that the blood be consumed and produced many times over. His ideas had remained unchallenged until the 17th century, and even then were defended by some physicians.[12]
In the mid 16th century Gabriele Falloppio (discoverer of the fallopian tubes) described what are now known as the lacteals as "coursing over the intestines full of yellow matter."[11] In about 1563 Bartolomeo Eustachi, a professor of anatomy, described the thoracic duct in horses as vena alba thoracis.[11] The next breakthrough came when in 1622 a physician, Gaspare Aselli, identified lymphatic vessels of the intestines in dogs and termed them venae alba et lacteae, which is now known as simply the lacteals. The lacteals were termed the fourth kind of vessels (the other three being the artery, vein and nerve, which was then believed to be a type of vessel), and disproved Galen's assertion that chyle was carried by the veins. But, he still believed that the lacteals carried the chyle to the liver (as taught by Galen).[13] He also identified the thoracic duct but failed to notice its connection with the lacteals.[11] This connection was established by Jean Pecquet in 1651, who found a white fluid mixing with blood in a dog's heart. He suspected that fluid to be chyle as its flow increased when abdominal pressure was applied. He traced this fluid to the thoracic duct, which he then followed to a chyle-filled sac he called the chyli receptaculum, which is now known as the cisternae chyli; further investigations led him to find that lacteals' contents enter the venous system via the thoracic duct.[11][13] Thus, it was proven convincingly that the lacteals did not terminate in the liver, thus disproving Galen's second idea: that the chyle flowed to the liver.[13] Johann Veslingius drew the earliest sketches of the lacteals in humans in 1647.[12]
The idea that blood recirculates through the body rather than being produced anew by the liver and the heart was first accepted as a result of works of William Harvey—a work he published in 1628. In 1652, Olaus Rudbeck (1630–1702), a Swede, discovered certain transparent vessels in the liver that contained clear fluid (and not white), and thus named them hepatico-aqueous vessels. He also learned that they emptied into the thoracic duct, and that they had valves.[13] He announced his findings in the court of Queen Christina of Sweden, but did not publish his findings for a year,[14] and in the interim similar findings were published by Thomas Bartholin, who additionally published that such vessels are present everywhere in the body, and not just the liver. He is also the one to have named them "lymphatic vessels".[13] This had resulted in a bitter dispute between one of Bartholin's pupils, Martin Bogdan,[15] and Rudbeck, whom he accused of plagiarism.[14]
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