Lymphatic vessels contain valves that prevent the backflow of transported lymph. The lymphatic vessels are so thin that the mere presence of valves gives the lymphatic channels a beaded appearance. Lymph flow from the peritoneum navigates through the thoracic duct to the intrathoracic lymph nodes.[rx] This extracellular fluid then returns to the bloodstream.[rx] Lymph is usually colorless, but that flowing from the intestinal organs is whitish (milky) due to the massive deposition of fat droplets within it and referred to as chyle. The lymphatic system in the gastrointestinal (GI) tract helps regulate the transport of chyle and balance interstitial fluid. A stimulant, such as feeding, activates lymph flow in the GI tract. It is also activated by cholecystokinin (CCK), glucagon, endothelin, bradykinin, substance-P, dopamine, serotonin, and many more. Lymph flow in the GI tract can also be inhibited by anti-diuretic hormone (ADH), vasoactive intestinal polypeptide (VIP), and acetylcholine. The lymphatic system in the intestines mainly functions to provide homeostasis in the GI tract by filtering fluids, blood cells, and plasma proteins that enter the tissue from the blood.[rx] The regions above and below the umbilicus drain into axillary lymph nodes and superficial inguinal nodes, respectively. Thus a watershed line is formed horizontally passing through the umbilicus where the lymphatic channels do not cross.[rx] The superficial inguinal nodes also receive lymph from the buttocks, penis, scrotum, labium majus, and the lower parts of the anal canal and vagina. This system eventually leads to the external iliac nodes, and finally, the lumbar aortic nodes. The pre-aortic nodes encompass the following nodes: celiac, superior mesenteric, and inferior mesenteric. These nodes drain lymph from the GI tract, spleen, gallbladder, pancreas, and liver. The para-aortic nodes, also known as the lumbar aortic nodes, drain lymph from the kidneys, suprarenal glands, testes, ovaries, uterus, and uterine tubes.
Lymphatic Vessel Structure
The lymphatic structure is based on that of blood vessels.
Key Points
Lymph (or lymphatic ) vessels are thin-walled valved structures that carry lymph.
Lymph vessels are lined by endothelial cells and have a thin layer of smooth muscles and adventitia that bind the lymph vessels to the surrounding tissue.
Lymph movement occurs despite low pressure due to smooth muscle action, valves, and compression during contraction of adjacent skeletal muscle and arterial pulsation.
When the pressure inside a lymphangion becomes high enough, lymph fluid will push through the semilunar valve into the next lymphangion, while the valve then closes.
Lymph vessels are structurally very similar to blood vessels.
Valves prevent backward flow of lymph fluid, which allows the lymphatic system to function without a central pump.
Key Terms
- lymphangial: The space between two semilunar valves of the lymphatic vessels that forms a distinct functional unit for the forward flow of lymph.
- adventitia: The outermost layer of connective tissue encasing a visceral organ or vessel.
- ISF: Interstitial (or tissue) fluid, a solution that bathes and surrounds the cells of multicellular animals. It is the main component of extracellular fluid, which also includes plasma and transcellular fluid.
- endothelial cells: A thin layer of cells that lines the interior surface of blood and lymphatic vessels, forming an interface between circulating blood or lymph in the lumen and the rest of the vessel wall.
The general structure of lymphatic vessels is similar to that of blood vessels since these are the only two types of vessels in the body. While blood and lymph fluid are two separate substances, both are composed of the same water (plasma or fluid) found elsewhere in the body.
Layers of Lymph Vessels
The endothelium, a general term for the inner layer of a vessel, is composed of an inner lining of single, flattened epithelial cells (simple squamous epithelium). This layer mechanically transports fluid. It sits on a highly permeable basement membrane made out of an extracellular matrix that separates the endothelium from the other layers. The endothelium is designed with junctions between cells that allow interstitial fluid to flow into the lumen when the pressure becomes high enough (such as from blood capillary hydrostatic pressure) but does not normally allow lymph fluid to leak back out into the interstitial space.
The next layer is smooth muscles arranged in a circular fashion around the endothelium that alter the pressure inside the lumen (space) inside the vessel by contracting and relaxing. The activity of smooth muscles allows lymph vessels to slowly pump lymph fluid through the body without a central pump or heart. By contrast, the smooth muscles in blood vessels are involved in vasoconstriction and vasodilation instead of fluid pumping.
The outermost layer is the adventitia, consisting of fibrous tissue. It is made primarily out of collagen and serves to anchor the lymph vessels to structures within the body for stability. Larger lymph vessels have many more layers of adventitia than do smaller lymph vessels. The smallest vessels, such as the lymphatic capillaries, may have no outer adventitia. As they proceed forward and integrate into the larger lymph vessels, they develop adventitia and smooth muscle. Blood vessels also have adventitia, sometimes referred to as tunica.
Lymphatic Valves
One of the main structural features of lymph vessels is their valves, which are semilunar structures attached to opposite sides of the lymphatic endothelium. Valves are found in larger lymph vessels and collecting vessels and are absent in the lymphatic capillaries. The valves is to prevent the backflow of fluid, so that lymph eventually flows forward instead of falling backward. When the pressure of lymph fluid increases to a certain point due to filling with more lymph fluid or from smooth muscle contraction, the fluid will be pushed through the valve (opening it) into the next chamber of the vessel (called a lymphangion). As the pressure falls, the open valve then closes so that the lymph fluid cannot flow backward.
Lymph Vessel: Diagram representing propulsion of lymph through a lymph vessel.
A lymphangion is the term for the space between two semilunar valves in a lymphatic vessel, the functional unit of the lymphatic system. Lymph fluid can only flow forward through lymphangitis due to the closing of valves after fluid is pushed through by fluid accumulation, smooth muscle contraction, or skeletal muscle contraction.
Without valves, the lymphatic system would be unable to function without a central pump. Smooth muscle contractions only cause small changes in pressure and volume within the lumen of the lymph vessels, so the fluid would just move backward when the pressure dropped. Blood vessels also have valves, but only in the low-pressure venous circulation. They function similarly to lymphatic valves, though are comparatively more dependent on skeletal muscle contractions.
Distribution of Lymphatic Vessels
The lymphatic system comprises a network of conduits called lymphatic vessels that carry lymph unidirectionally towards the heart.
Key Points
The lymph system is not a closed system. Lymph flows in one direction toward the heart.
Lymph nodes are most densely distributed toward the center of the body, particularly around the neck, intestines, and armpits.
Lymph vessels and nodes are not found within the bone or nervous system tissue.
Afferent lymph vessels flow into lymph nodes, while efferent lymph vessels flow out of them.
Lymphatic capillaries are the sites of lymph fluid collection and are distributed throughout most tissues of the body, particularly connective tissue.
Key Terms
- lymph: A colorless, watery, bodily fluid carried by the lymphatic system, consisting mainly of white blood cells.
- plasma: The straw-colored/pale-yellow liquid component of blood that normally holds the blood cells of whole blood in suspension.
- Efferent: A type of vessel that flows out of a structure, such as lymph vessels that leave the spleen or lymph nodes and arterioles that leave the kidney.
The lymphatic system is a circulatory system for lymphatic fluid, comprising a network of conduits called lymphatic vessels that carry the fluid in one direction toward the heart. Its functions include providing sites for certain immune system functions and facilitating plasma circulation in the cardiovascular system. The lymphatic system is composed of many different types of lymph vessels over a wide distribution throughout the body.
Lymph Node Distribution
Lymphatic System: The lymph nodes and lymph vessels in human beings.
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.
- Afferent lymphatic vessels flow into a lymph node and carry unfiltered lymph fluid.
- Efferent lymphatic vessels flow out of a lymph node and carry filtered lymph fluid. Lymph vessels that leave the thymus or spleen (which lack afferent vessels) also fall into this category.
Lymph nodes are most densely distributed around the pharynx and neck, chest, armpits, groin, and 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.
Flow-Through Lymph Vessels
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 interstitial 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.
Like cardiovascular capillaries, lymph capillaries are well distributed throughout most of the body’s tissues, though they are mostly absent in bone or nervous system tissue. 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 a linear flow, while the cardiovascular system is a closed system with the 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 adventitia 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 Transport
Lymph circulates to the lymph node via afferent lymphatic vessels and drains into the lymph node in the subcapsular sinus.
Key Points
The sinus space is crisscrossed by the pseudopods of macrophages, which act to trap foreign particles and filter the lymph.
Lymph then leaves the lymph node via the efferent lymphatic vessel towards either a more central lymph node or for drainage into a central venous subclavian blood vessel.
Lymphatic transport begins in the lymphatic capillaries, which converge into collecting vessels that flow into afferent vessels, then into lymph nodes.
The lymph fluid leaves the node through efferent lymph vessels, which converge into lymphatic trunks, which in turn converge into one of the lymphatic ducts that flow lymph back into the venous circulation.
B and T lymphocytes must be transported to different sites within lymph nodes during an adaptive immune response.
Key Terms
- afferent lymphatic vessels: These vessels enter into the lymph nodes, flowing into the sinus space below the capsule of the node.
- lymph: A colorless, watery bodily fluid carried by the lymphatic system, consisting mainly of white blood cells.
- germinal centers: Places within secondary lymph nodes to which B cells migrate to proliferate and differentiate based on an antigen response.
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 the venous circulation.
Transport in the Lymph Capillaries and Vessels
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 mini valves located in the junctions of the endothelium. Under ordinary conditions, these mini valves 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 the 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.
Transport Within Lymph Nodes
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 the 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.
The End of Lymphatic Transport
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.
Lymphatic Capillaries
Lymph capillaries are tiny, thin-walled vessels, closed at one end and located in the spaces between cells throughout the body.
Key Points
Lymph or lymphatic capillaries are tiny thin-walled vessels, closed at one end and located in the spaces between cells throughout the body, except in the central nervous system and non-vascular tissues.
Lymphatic capillaries are slightly larger in diameter and have greater oncotic pressure than blood capillaries.
When pressure is greater in the interstitial fluid than in lymph, the mini valve cells separate slightly and interstitial fluid enters the lymphatic capillary. When pressure is greater inside the lymphatic capillary, the cells of the mini valves adhere more closely, and lymph cannot flow back into the interstitial fluid.
Anchoring filaments attach to the mini valves to anchor the capillary to connective tissue and also pull the capillary open to increase lymph collection when the tissue is swollen.
Because lymph capillaries have a closed-end, lymph is pushed forward into larger vessels as the pressure inside the capillary increases as lymph accumulates from fluid collection.
Edema can occur when interstitial fluid accumulation in tissues is greater than fluid removal (acute inflammation ) or when the lymph vessels are obstructed in some way (elephantiasis).
Key Terms
- interstitial fluid: Also called tissue fluid, a solution that bathes and surrounds the cells of multicellular animals.
- lymph capillaries: Tiny thin-walled vessels, closed at one end and located in the spaces between cells throughout the body, collect fluid from the tissues.
Lymphatic circulation begins in the smallest type of lymph vessels, the lymph capillaries. These regulate the pressure of the interstitial fluid by draining lymph from the tissues.
Structure of Lymphatic Capillaries
Lymph or lymphatic capillaries are tiny thin-walled vessels, closed at one end and located in the spaces between cells throughout the body. These are particularly dense within connective tissue. Lymphatic capillaries are slightly larger in diameter than blood capillaries and contain flap-like “mini valves” that permit interstitial fluid to flow into them but not out, under normal conditions.
Lymphatic capillaries are primarily made out of an endothelium layer that sits on a permeable basement membrane. The flap-like mini valves, located at gap-like junctions in the endothelium, are formed from the overlap of endothelial cells and are normally closed. Attached to the outer opening of the mini valves are anchoring filaments containing elastic fibers. They extend out from the lymphatic capillary, attaching the endothelium to fibroblast cells in the connective tissue. Unlike larger lymphatic vessels, lymphatic capillaries do not contain smooth muscle nor do they have a well-developed adventitia, only small elastic filaments that perform a similar function.
The function of Lymphatic Capillaries
The lymph capillaries serve a variety of important functions.
Fluid Pressure Regulation
Lymphatic capillaries collect lymph fluid from the tissues, which allows them to regulate the pressure of the interstitial fluid. This fluid is essentially plasma that leaks out of cardiovascular capillaries into the tissues due to the forces of hydrostatic or oncotic pressure. When pressure is greater in the interstitial fluid than in lymph due to the accumulation of interstitial fluid, the mini valves separate slightly like the opening of a one-way swinging door so that fluid can enter the lymphatic capillary. When pressure is greater inside the lymphatic capillary, the cells adhere more closely to each other to prevent lymph backflow. The anchoring filaments are also pulled when the tissues are swollen. This opens the lymph capillaries more, increasing their volume and reducing their pressure to further facilitate fluid flow into the capillaries.
Lymph capillaries have a greater oncotic pressure (a pulling pressure exerted by proteins in solution) than blood plasma due to the greater concentration of plasma proteins in lymph. Additionally, the greater size of lymphatic capillaries compared to cardiovascular capillaries allows them to take more fluid proteins into lymph compared to plasma, which is the other reason for their greater levels of oncotic pressure. This also explains why lymph flows into the lymph capillaries easily, since fluid follows proteins that exert oncotic pressure.
Edema Prevention
Under normal conditions, lymph capillaries prevent the accumulation of edema (abnormal swelling) in the tissues. However, edema will still occur during acute inflammation or diseases in which lymph vessels are obstructed. During inflammation, fluid leaks into the tissues at a rate faster than it can be removed by the lymph capillaries due to the increased permeability of cardiovascular capillaries. During lymph vessel obstruction (such as through elephantiasis infection), lymph will be unable to progress normally through the lymphatic system, and pressure within the blocked-off lymph capillaries increases to the point where backflow into tissues may occur, while the pressure of interstitial fluid gradually rises.
Drive Lymph Through Lymphatic Vessels
Lymph Capillary: Diagram showing the formation of lymph from interstitial fluid (labeled here as “Tissue fluid”). Note: how the tissue fluid is entering the blind ends of lymph capillaries (indicated by deep green arrows).
The lymphatic capillaries bring lymph further into the lymphatic vessels. The capillaries have external valves but no internal valves or smooth muscle, so the pressure of lymph accumulation itself must propel the fluid forward into the larger vessels. Because lymphatic capillaries have closed-end and mini valves normally prevent backflow into tissues, the pressure of lymph becomes higher as more lymph is collected from the tissues, which sends the lymph fluid forward. Multiple capillaries converge in collecting vessels, where the internal valves and smooth muscle start to appear. This moves lymph further along the system despite the fall in pressure that occurs when moving from the higher-pressure capillaries to the lower-pressure collecting vessels.
Lymph Trunks and Ducts
The lymph trunks drain into the lymph ducts, which in turn return lymph to the blood by emptying into the respective subclavian veins.
Key Points
The lymph trunks drain into the lymph ducts, which in turn return lymph to the blood by emptying into the respective subclavian veins.
There are two lymph ducts in the body: the right lymph duct and the thoracic duct.
There are four pairs of lymph trunks: jugular lymph trunks, subclavian lymph trunks, bronchomediastinal lymph trunks, and lumbar lymph trunks. In addition, the intestinal lymph trunk is unpaired.
The intestinal lymph trunk and the thoracic lymph duct contain chyle, a mixture of emulsified fats from the intestines and lymph fluid.
Key Terms
- thoracic duct: The lymph duct that drains lymph and chyle from the lower and left halves of the body.
- subclavian vein: Two large veins, one on either side of the body, with a diameter similar to that of the smallest finger.
- lymph: A colorless, watery body fluid carried by the lymphatic system, consisting mainly of white blood cells.
After filtration by the lymph nodes, efferent lymphatic vessels take lymph to the end of the lymphatic system. The final goal of the lymphatic system is to recirculate lymph back into the plasma of the bloodstream. There are two specialized lymphatic structures at the end of the lymphatic system, called the lymph trunks and ducts.
Lymphatic Trunks
Lymphatic Ducts: The thoracic duct and right lymphatic duct.
A lymphatic trunk is any large lymph vessel that forms from the convergence of many efferent lymph vessels. There are four sets of lymph trunks that are paired with a right and left half and one unpaired trunk:
- Jugular lymph trunks, located in the neck, drain lymph fluid from the cervical lymph nodes of the neck.
- Subclavian lymph trunks, located beneath the clavicle, drain lymph fluid from the apical lymph nodes around the armpit, which carry lymph from the arms.
- Bronchomediastinal lymph trunks, located in the chest, drain lymph fluid from the lungs, heart, trachea, mediastinal, and mammary glands.
- Lumbar lymph trunks are the lower pair of lymph trunks that drain lymph fluid from the legs, pelvic region, and kidneys.
- The intestinal lymph trunk is the unpaired lymph trunk that receives chyle (lymph mixed with fats) from the intestines. Chyle typically has a high fatty acid content.
Lymphatic trunks then drain lymph fluid into the lymph ducts, the final part of the lymphatic system.
Lymph Ducts
Two lymph ducts receive lymph from the lymph trunks. These are the largest lymph vessels and contain three layers, similar to those of great veins.
- The thoracic lymph duct, the largest lymph vessel in the body, takes lymph from the lower and left halves of the body. Because the thoracic lymph duct drains the intestinal lymph trunks, it carries a mixture of lymph and emulsified fatty acids called chyle back to the bloodstream.
- The right lymphatic duct receives lymph from the right and upper halves of the body, including the right sides of the jugular, bronchomediastinal, and subclavian lymph trunks.
The thoracic duct drains into to the left subclavian vein while the right duct drains into the right subclavian vein, both at the junction between the respective vein and the jugular vein. The two subclavian veins then merge into the vena cava, the large vein that brings deoxygenated blood to the heart. The lymph ducts each have internal valves at their junction with the subclavian vein. These function similarly to other lymphatic valves and prevent venous blood from flowing into the lymph duct. This point marks the end of lymph fluid’s journey through the lymphatic system.
Comparison of blood vascular endothelial cells (BECs) and lymphatic endothelial cells (LECs)
| Feature | Blood vessels/BEC | Lymphatics/LEC |
|---|---|---|
| Constituents | Blood, blood cells | Lymph (interstitial fluid rich in protein, fat, and lipids, extravasated immune cells, and large extracellular molecules) |
| Gross structure | Closed, circular | Open, linear |
| Start/end | Heart/heart | Tissue/lymph-vein connection of the thoracic duct |
| Hierarchical division | Arteries, arterioles, capillaries, venules, veins | Capillaries, precollectors, collecting vessels, thoracic duct, lymph nodes |
| Vessel wall | Adherens and tight junctions, continuous basement membrane, pericytes, or vascular smooth muscle cells | Overlapping LECs, no tight junctions, anchoring filaments, discontinuous basement membrane, few pericytes (collecting lymphatic vessels have both continuous membranes and mural cells) |
| Development | Vasculogenesis and angiogenesis | Lymphangiogenesis (budding from cardinal vein) |
| Origin | Mesoderm, endothelial stem/precursor cells from bone marrow for adults | Mesoderm (vein) during development, lymphatic progenitor cells from bone marrow for adults |
| Examples of cell type–specific markers | CD34, CD105/endoglin | Prox1, LYVE-1, VEGFR-3, and podoplanin |
| Absence | Cartilage, cornea | Cartilage, brain, bone, spinal cord, and the retina |
| Functions | Hemostasis, inflammation, leukocyte trafficking, barrier function, delivery for oxygen, nutrients, and tissue wastes | Tissue fluid homeostasis, absorption of large molecules and lipids in the digestive systems, trafficking of lymphocytes and antigen-presenting cells to regional lymph nodes, transport of degraded extracellular molecules, cell debris, and lymph fluid |
| Heterogeneity | Well-established phenotypic heterogeneity | Comparable LEC heterogeneity was reported. LEC fate is highly plastic in response to genetic and environmental stimuli |
References
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