122 Lymphatic Vessels

Lymphatic Vessel Structure

The lymphatic structure is based on that of blood vessels.

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 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 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 alters 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 backflow of fluid, so that lymph eventually flows forward instead of falling backwards. When the pressure of lymph fluid increase 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 backwards.

A lymphangion is the term for the space between two semilunar valves in a lymphatic vessel, functional unit of the lymphatic system. Lymph fluid can only flow forward through lymphangions 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 backwards when the pressure dropped. Blood vessels also have valves, but only in 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.

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 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 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.

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 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.

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 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 Transport

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.

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 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.

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 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.

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.

Lymphatic circulation begins in the smallest type of lymph vessels, the lymph capillaries. These regulate the pressure of 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 “minivalves” 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 minivalves, 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 minivalves 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.

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 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 accumulation of interstitial fluid, the minivalves 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

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 a closed end and minivalves 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.

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