The lymphatic system is a network of tissues, vessels and organs that work together to move a colorless, watery fluid called lymph back into your circulatory system (your bloodstream). Some 20 liters of plasma flow through your body’s arteries and smaller arteriole blood vessels and capillaries every day.
The lymphatic system is a network of tissues and organs that help rid the body of toxins, waste and other unwanted materials. The primary function of the lymphatic system is to transport lymph, a fluid containing infection-fighting white blood cells, throughout the body.
The lymphatic system primarily consists of lymphatic vessels, which are similar to the veins and capillaries of the circulatory system. The vessels are connected to lymph nodes, where the lymph is filtered. The tonsils, adenoids, spleen, and thymus are all part of the lymphatic system.
What are the parts of the lymphatic system?
The lymphatic system consists of many parts. These include:
- Lymph: Lymph, also called lymphatic fluid, is a collection of the extra fluid that drains from cells and tissues (that is not reabsorbed into the capillaries) plus other substances. The other substances include proteins, minerals, fats, nutrients, damaged cells, cancer cells, and foreign invaders (bacteria, viruses, etc). Lymph also transports infection-fighting white blood cells (lymphocytes).
- Lymph nodes: Lymph nodes are bean-shaped glands that monitor and cleanse the lymph as it filters through them. The nodes filter out the damaged cells and cancer cells. These lymph nodes also produce and store lymphocytes and other immune system cells that attack and destroy bacteria and other harmful substances in the fluid. You have about 600 lymph nodes scattered throughout your body. Some exist as a single node; others are closely connected groups called chains. A few of the more familiar locations of lymph nodes are in your armpit, groin, and neck. Lymph nodes are connected to others by the lymphatic vessels.·
- Lymphatic vessels: Lymphatic vessels are the network of capillaries (microvessels) and a large network of tubes located throughout your body that transport lymph away from tissues. Lymphatic vessels collect and filter lymph (at the nodes) as it continues to move toward larger vessels called collecting ducts. These vessels operate very much as your veins do: They work under very low pressure, have a series of valves in them to keep the fluid moving in one direction.
- Collecting ducts: Lymphatic vessels empty the lymph into the right lymphatic duct and left lymphatic duct (also called the thoracic duct). These ducts connect to the subclavian vein, which returns lymph to your bloodstream. The subclavian vein runs below your collarbone. Returning lymph to the bloodstream helps to maintain normal blood volume and pressure. It also prevents the excess buildup of fluid around the tissues (called edema).
The lymphatic system collects excess fluid that drains from cells and tissue throughout the body and returns it to the bloodstream, which is then recirculated through the body.
- Spleen – This largest lymphatic organ is located on your left side under your ribs and above your stomach. The spleen filters and stores blood and produces white blood cells that fight infection or disease.
- Thymus – This organ is located in the upper chest beneath the breast bone. It matures a specific type of white blood cell that fights off foreign organisms.
- Tonsils and adenoids – These lymphoid organs trap pathogens from the food you eat and the air you breathe. They are your body’s first line of defense against foreign invaders.
- Bone marrow – This is the soft, spongy tissue in the center of certain bones, such as the hip bone and breastbone. White blood cells, red blood cells, and platelets are made in the bone marrow.
- Peyer’s patches – These are small masses of lymphatic tissue in the mucous membrane that lines your small intestine. These lymphoid cells monitor and destroy bacteria in the intestines.
- Appendix – Your appendix contains lymphoid tissue that can destroy bacteria before it breaches the intestine wall during absorption. Scientists also believe the appendix plays a role in housing “good bacteria” and repopulating our gut with good bacteria after an infection has cleared.
Structure of the Lymphatic System
The lymphatic system consists of lymphatic vessels and associated lymphoid organs.
Key Points
The lymphatic system is a circulatory system that drains fluid from the blood vessels.
Lymph vessels are the site of fluid drainage and pump lymph fluid using smooth muscle and skeletal muscle action. The larger vessels contain valves to prevent backflow and pump towards the heart to return lymph fluid to the bloodstream by the subclavian veins.
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.
Lymphoid tissue contains lymphocytes and other specialized cells and tissues that have immune system functions.
Key Terms
- lymph node: Small oval bodies of the lymphatic system, distributed along the lymphatic vessels clustered in the armpits, groin, neck, chest, and abdomen. They filter through lymph fluid.
- lymph: A colorless, watery, bodily fluid carried by the lymphatic system, consisting mainly of white blood cells.
The lymphatic system is a collection of structures and vessels that drains lymph from blood and has several other functions. It is a circulatory system for lymph fluid and the site of many key immune system functions.
Lymphatic Vessels
The lymphatic vessels are the lymphatic system equivalent to the blood vessels of the circulatory system and drain fluid from the circulatory system. The network of lymph vessels consists of the initial collectors of lymph fluid, which are small, valveless vessels, and goes on to form the collector vessels, which have rudimentary valves that are not fully functional. These structures then form increasingly larger lymphatic vessels which form collaterals and have lymph regions (lymph hearts). The larger lymph vessels contain valves that prevent the backflow of lymph.
The lymphatic system is an active pumping system driven by segments that have a function similar to peristalsis. They lack a central pump (like the heart in the cardiovascular system), so smooth muscle tissue contracts to move lymph along through the vessels. Skeletal muscle contractions also move lymph through the vessels. The lymphatic vessels make their way to the lymph nodes, and from there the vessels form into trunks. In general, the lymph vessels bring lymph fluid toward the heart and above it to the subclavian veins, which enable lymph fluid to re-enter the circulatory system through the vena cava.
Lymphatic Tissues and Organs
Lymphoid tissue is found in many organs including the lymph nodes, as well as in the lymphoid follicles in the pharynx such as the tonsils. Lymph nodes are found primarily in the armpits, groin, chest, neck, and abdomen. Lymphoid tissues contain lymphocytes (a type of highly differentiated white blood cell), but they also contain other types of cells for structural and functional support, such as the dendritic cells, which play a key role in the immune system. The system also includes all the structures dedicated to the circulation and production of lymphocytes, including the spleen, thymus, and bone marrow.
Functions of the Lymphatic System
The lymphatic system plays a prominent role in immune function, fatty acid absorption, and removal of interstitial fluid from tissues.
Your lymphatic system has many functions. Its key functions include:
- Maintains fluid levels in your body: As just described, the lymphatic system collects excess fluid that drains from cells and tissue throughout your body and returns it to your bloodstream, which is then recirculated through your body.
- Absorbs fats from the digestive tract: Lymph includes fluids from your intestines that contain fats and proteins and transports it back to your bloodstream.
- Protects your body against foreign invaders: The lymphatic system is part of the immune system. It produces and releases lymphocytes (white blood cells) and other immune cells that monitor and then destroy the foreign invaders — such as bacteria, viruses, parasites and fungi — that may enter your body.
- Transports and removes waste products and abnormal cells from the lymph.
Key Points
The lymphatic system is a linear network of lymphatic vessels and secondary lymphoid organs. It is the site of many immune system functions as well as its own functions.
It is responsible for the removal of interstitial fluid from tissues into lymph fluid, which is filtered and brought back into the bloodstream through the subclavian veins near the heart.
Edema accumulates in tissues during inflammation or when lymph drainage is impaired.
It absorbs and transports fatty acids and fats as chylomicrons from the digestive system.
It transports white blood cells and dendritic cells to lymph nodes where adaptive immune responses are often triggered.
Tumors can spread through lymphatic transport.
Key Terms
- lacteal: A lymphatic capillary that absorbs dietary fats in the villi of the small intestine.
- interstitial fluid: Also called tissue fluid, a solution that bathes and surrounds the cells of multicellular animals.
- white blood cell: A type of blood cell involved with an immune response. Many white blood cells (primarily lymphocytes) are transported by the lymphatic system.
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.
Removal of Fluid
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.
The lymphatic system: A diagram of fluid movement in the lymphatic system.
Fatty Acid Transport
The lymphatic system also facilitates fatty acid absorption from the digestive system. During fat digestion, fatty acids are digested, emulsified, and converted within intestinal cells into a lipoprotein called chylomicrons. Lymph drainage vessels that line the intestine, called lacteals, absorb the chylomicrons into lymph fluid. The lymph vessels then take the chylomicrons into blood circulation, where they react with HDL cholesterols and are then broken down in the liver.
Immune Cell Transport
In addition to tissue fluid homeostasis, the lymphatic system serves as a conduit for transport of cells involved in immune system function. Most notably, highly-specialized white blood cells called lymphocytes and antigen -presenting cells are transported to regional lymph nodes, where the immune system encounters pathogens, microbes, and other immune elicitors that are filtered from the lymph fluid. Much of the adaptive immune system response, which is mediated by dendritic cells, takes place in the lymph nodes. Lymphatic vessels, which uptake various antigens from peripheral tissues, are positively regulated by chemokines/cytokines secreted by various immune cells during inflammation. This allows antigens to enter lymph nodes, where dendritic cells can present them to lymphocytes to trigger an adaptive immune response.
While the lymphatic system is important for transporting immune cells, its transport capabilities can also provide a pathway for the spread of cancer. Lymph circulation is one of the main ways that tumors can spread to distant parts of the body, which is difficult to prevent.
Blood Supply and Lymphatics
Lymph Nodes of the Head and Neck
The lymph nodes in the head and neck are paired and broadly split into superficial and deep nodes.
Superficial
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Occipital nodes – At the lateral border of the trapezius muscles
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These drain the skin overlying the occiput.
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Mastoid (post-auricular) nodes – At the insertion of the sternocleidomastoid muscle on the mastoid process of the temporal bone
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These drain the posterior neck, the superior portion of the external ear, and the ear canal until the tympanic membrane.
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Pre-auricular nodes – Anterior to the tragus of the ear
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These drain the superficial face and temporal region.
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Superficial parotid nodes – Overlying the parotid gland.
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This drain the nose, the nasal cavity, part of the external ear canal, and the lateral orbit
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Submental nodes – Overlying the mylohyoid muscle.
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These drain the middle-lower lip, the floor of the mouth, and the tip of the tongue.
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Submandibular nodes – Found in the submandibular triangle, bounded by the inferior edge of the mandible and the anterior and posterior bellies of the digastric muscle, overlying the mylohyoid and hyoglossus muscles
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These drain the submental and facial nodes, the cheeks, the upper lip, and the marginal areas of the lower lip.
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Facial nodes – Comprised of maxillary, buccinator, and submandibular lymph nodes
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These drain the mucus membranes of the inside of the cheek, the nasal mucosa, the eyelids, and the conjunctiva.
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Superficial cervical
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Anterior superficial cervical – Along the anterior jugular vein.
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These drain the superficial portions of the anterior neck.
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Posterior superficial cervical – Along the external jugular vein
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These also drain the superficial tissues of the neck.
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Deep
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Deep parotid – Found deep to the parotid gland.
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These drain the nasal cavity and the nasopharynx.
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Deep cervical – Found along the internal jugular vein. These are named prelaryngeal, pretracheal, retropharyngeal, infrahyoid, jugulodigastric, jugulo-omohyoid, and supraclavicular nodes, depending on their anatomical positional.
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These drain the superficial nodes and all of the head and neck.
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In terms of anatomical dissection, these more easily split into levels of the neck.[rx]
Level I
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Level Ia (submental nodes) – anteriorly, in the midline between the anterior bellies of the paired digastric muscles
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Level Ib (submandibular nodes) – in the submandibular triangle, as described above.
Level II
These nodes, also called the upper internal jugular nodes, are found in an anatomical area bounded by the base of the skull superiorly, the hyoid bone inferiorly, the submandibular gland anteriorly, the posterior border of the sternocleidomastoid muscle laterally, and the internal carotid artery medially. The spinal accessory nerve separates the level IIa and IIb nodes.
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Level IIa (jugulo-digastric nodes) – superficial or anterior to internal jugular vein
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Level IIb – deep or posterior to the internal jugular vein
Level III
These nodes are also names the middle internal jugular nodes and are bound superiorly by the hyoid bone, the cricoid cartilage inferiorly, the anterior edge of the sternocleidomastoid muscle anteriorly, the posterior margin of the sternocleidomastoid muscle, and the common carotid artery medially.
Level IV
These nodes are also named the lower internal jugular nodes and include Virchow’s node. The anatomical area in which they are found is bound superiorly by the cricoid cartilage, the clavicle inferiorly, the sternocleidomastoid muscle anteriorly, and the common carotid artery medially.
Level V
These are also named the posterior triangle nodes and are bounded by the convergence of the sternocleidomastoid and trapezius muscles superiorly, the clavicle inferiorly, the sternocleidomastoid muscle anteriorly and medially, and the trapezius muscle posteriorly.
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Level Va – superior to the cricoid cartilage and include the spinal accessory nodes
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Level Vb – inferior to the cricoid cartilage and include the transverse cervical nodes and the supraclavicular nodes
Level VI
This level is also named the anterior compartment and contains the anterior jugular, pre-tracheal, para-tracheal, pre-cricoid, pre-laryngeal, and thyroid nodes. It is bound superiorly by the hyoid bone, inferiorly by the suprasternal notch, by the platysma muscle anteriorly, and the common carotid artery laterally.
Lymph Nodes of the Upper Limb
The deep and superficial lymphatics in the upper limb eventually drain into the axillary nodes. However, there are supratrochlear and cubital lymph nodes at the level of the elbow, brachial lymph nodes, and deltopectoral lymph nodes. The drainage of the upper limbs is particular due to the presence of sentry or sentinel lymph nodes. These are usually larger than the rest of the lymph nodes and are the first to filter the incoming lymph. However, it is not uncommon for multiple smaller sentry lymph nodes to also be present.[rx][rx]
Axillary Nodes
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Anterior nodes (pectoral) – They drain the breast and the anterior thoracic wall.
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Posterior nodes (subscapular) – They drain the posterior thoracic wall and the scapular area.
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Lateral nodes (humeral) – These are found posterior to the axillary vein and are the primary draining nodes for the upper limb.
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Central nodes – These are found close to the 2nd part of the axillary artery and receive lymph from the anterior, posterior, and lateral nodes.
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Apical nodes – These are located near the 1st part of the axillary artery and vein and filter the lymph received from the central axillary nodes and the cephalic vein.
The apical nodes further form the subclavian lymphatic trunk, which then drains into the right lymphatic duct.
Lymph Nodes of the Lower Limb
The superficial and deep lymphatic vessels of the lower limb drain into the inguinal lymph nodes in the femoral triangle. This anatomical region, also named Scarpa’s triangle, is bounded by the inguinal ligament above, the medial border of the sartorius muscle laterally, and the medial border of the adductor longus muscle medially.[rx][rx]
Inguinal Nodes
The inguinal lymph nodes split at the level (where the great saphenous vein becomes the deep femoral vein) into sub-inguinal lymph nodes below and superficial inguinal nodes above.[rx]
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Sub-inguinal nodes
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Superficial sub-inguinal nodes – These are found alongside the proximal saphenous vein and drain the superficial lymphatic vessels.
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Deep sub-inguinal nodes – These nodes are commonly found alongside the medial femoral vein and collect lymph from the deep lymphatic channels of the lower limb.
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Superficial inguinal nodes – These nodes are traditionally found immediately inferior to the inguinal ligament and drain the perineal area (penis, scrotum, perineum), the gluteal region, and part of the abdominal wall.
Iliac Nodes:[rx]
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External iliac nodes
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Lateral external iliac lymph nodes – These are found lateral to the external iliac artery.
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Intermediate external iliac lymph nodes – These are found medial to the external iliac artery and anterior to the external iliac vein.
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Medial external iliac lymph nodes – These are found medial to the external iliac vein.
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Common iliac nodes – They commonly arise at the level of the aortic bifurcation (4th lumbar vertebra) and extend until the level of the common iliac bifurcation (2nd sacral vertebra)
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Lateral common iliac lymph nodes – These are found lateral to the common iliac artery.
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Intermediate common iliac lymph nodes – These appear alongside the posteromedial common iliac artery.
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Medial common iliac lymph nodes – These are alongside the medial common iliac artery.
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Terms and Definitions
| Breast Cancer Associated Lymphœdema: | Pathologic swelling of the arm as an eventual result of lymph node removal from the axillary region as part of mastectomy procedures. Lymph nodes are removed as part of the surgery to determine and/or minimize the probability that cancer has spread to other tissues, this being an important prognostic indicator. |
| Chemokine | Cytokines (cell signalling proteins) that are specific instigators to immune cell migration in peripheral tissues as well as lymph nodes. There are some 45 chemokines and 20 chemokine receptors identified so far. |
| Collecting Lymphatic | A medium- to a large-sized lymphatic vessel having an inner lining of lymphatic endothelial cells, an outer lining of several layers of lymphatic muscle cells, and interspersed elastin (inner layers) and collagen (outer layers). Collecting lymphatic vessels also have bi-leaflet one-way valves at regular, short intervals. These vessels exhibit a highly nonlinear pressure-diameter behavior. |
| Decongestive Lymphatic Therapy | A combination of medical procedures intended to minimize the further progression of Breast Cancer-Associated Lymphœdema. Includes skin care, compression bandaging, exercise, and specialized massage (manual lymphatic drainage). |
| Extrinsic Pumping | Results from lymphatic vessel compression due to movement of adjacent tissues. |
| Immune System | A major bodily function spanning several organs and systems that defends against disease. Key cell types are dendritic cells, macrophages, B cells and T cells. |
| Initial Lymphatic | The smallest of the lymphatic vessels, they take in interstitial fluid. Consists of a layer of lymphatic endothelial cells that are tethered to surrounding tissues. Sometimes called “lymphatic capillaries” or “terminal lymphatics.” |
| Interstitium | The spaces between cells and tissue-specific structures such as barrier membranes. Contains fluids, proteins, etc. that are taken up into initial lymphatics. |
| Intrinsic Pumping | Results from active contraction of lymphatic muscle cells in the walls of collecting lymphatic vessels. |
| Lumped-Parameter Modelling | Use of 0D models of fluid flow variables in the equations of motion and constitutive relations. When a lumped-parameter model consists of many segments, it is not essentially different from a finite-difference solution of 1D equations. |
| Lymph | The fluid flows through lymphatic vessels. Contains mainly water, with suspended proteins and immune cells. |
| Lymph Node | Small (1−2 cm or less), kidney-shaped organs central to immune system function. Lymph from peripheral tissues is pumped into lymph nodes by afferent collecting lymphatic vessels. Pathogens are then filtered and screened by immune cells within the node. Fluid and immune cells move across the walls of specialized blood vessels, which include high endothelial venules. There are some 500−600 lymph nodes in the human body. |
| Nitric Oxide | A powerful vasodilator substance first identified in blood vessels. Also secreted by lymphatic endothelial cells as a trigger for lymphatic muscle cells to dampen contraction. |
References
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