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. 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. 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.
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 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.
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 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 of lymph trunks that are paired with a right and left half, and one unpaired trunk:. Lymphatic trunks then drain lymph fluid into the lymph ducts, the final part of the lymphatic system. 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 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. Privacy Policy. Skip to main content. Lymphatic System. Search for:. Lymphatic Vessels. Lymphatic Vessel Structure The lymphatic structure is based on that of blood vessels.
Learning Objectives Describe lymphatic circulation and the structure of lymphatic vessels. Key Takeaways 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 backwards flow of lymph fluid, which allows the lymphatic system to function without a central pump. Key Terms lymphagion : The space between two semilunar valves of the lymphatic vessels that forms a distinct functional unit for the forward flow of lymph.
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. Distribution of Lymphatic Vessels The lymphatic system comprises a network of conduits called lymphatic vessels that carry lymph unidirectionally towards the heart. Learning Objectives Describe the structure of the lymphatic system and its role in the immune system and blood circulation.
Key Takeaways Key Points The lymph system is not a closed system. 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 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. It consists mainly of a substance called hyaluronan which is a clear fluid made up of sugars, but also contains proteins.
The protein concentration of lymph changes depending on the location of lymph in the lymphatic system. Interstitial fluid fluid entering the lymphatic system and afferent lymph lymph which has not yet passed through a lymph node contain 20—30 grams of protein per litre, whereas efferent lymph lymph which has passed through a lymph node contains 60 grams of protein per litre. Afferent lymph is that which has been absorbed by the lymphatic capillaries the smallest of the lymphatic vessels and the entry point to the lymphatic system but has not yet passed through a lymph node for filtration.
It may contain erythrocytes red blood cells , monocytes white blood cells which support the immune system , and dendritic cells also called antigen presenting cells which have leaked from the blood vessels into the interstitial spaces and been absorbed by the lymphatic capillaries.
These components are not typically found in efferent lymph. Afferent lymph also contains lymphocytes, macrophages and debris from dead cells. Macrophages are cells of the immune system which can scavenge for foreign particles like bacteria, and then engulf and eat those foreign particles. However, it contains very low levels of macrophages.
The additional lymphocytes enter the lymph at lymph nodes via tiny vessels which enable the transport of lymphocytes directly from the bloodstream to the lymph node. The tiny vessels which transport the lymphocytes have specialised walls which allow greater concentrations of lymphocytes entry to the lymph compared to other tissues. The collections of lymphocytes which enter efferent lymph directly from the blood via the lymph nodes are active and completely functional.
They continuously recirculate between blood, lymph and other tissues. They extend throughout the body, typically in close association with blood vessels.
Most parts of the body through which blood circulates, also have lymphatic vessels which transport lymph. Exceptions include the central nervous system , brain , retina and cornea of the eye , epidermis skin , hair, nails and cartilage. While there are no lymphatic vessels in the brain or central nervous system, the lymphatic vessels are connected to the neurological system via nerves.
Lymphatic capillaries also called initial lymphatics are microscopic vessels which form web-like networks in the interstitial spaces spaces between body organs and tissues. They are the entry-point of lymph to the lymphatic system and found in the interstitial spaces surrounding most tissues. Lymphatic capillaries are similar in appearance to blood capillaries. The wall of each lymphatic capillary consists of a single layer of cells which are loosely connected.
Cells of the lymphatic capillary walls connect to each other in an arrangement in which they loosely overlap, forming flap-like structures. The flap-like structures have a similar appearance to valves, and are sometimes also called microvalves. Lymphatic capillaries are blind-ended their ends are closed ; however, the flap-like structures of the capillaries walls make the closed ends highly permeable to relatively large molecules, including antigens like viruses and bacteria.
The flap-like structures of the capillary wall connect directly to the surrounding structures organs or tissues , via thin, elastic fibres. The fibres connect only to the external surface of the capillary wall, leaving the internal surface unattached and able to move.
While lymphatic capillaries are found throughout the body, they are more extensive in the legs compared to the arms. Their concentration is dynamic and increases at times of inflammation, when too much interstitial fluid is in the interstitial spaces.
The interstitial fluid must be absorbed into the lymphatic system to reduce inflammation, and in order to ensure the fluid is absorbed efficiently, new lymphatic capillaries grow in a process known as lymphangiogenesis.
Lymphangiogenesis enables the proliferation of lymphatic capillaries at the inflamed site resulting in more efficient drainage of fluid from the interstitial spaces.
Lymphatic collecting vessels run throughout the body and connect to lymphatic capillaries. They grow successively larger as they increase their distance from the capillaries and reduce their distance from the heart. As they move towards the heart, lymphatic collecting vessels pass through thousands of lymph nodes which filter the lymph.
The collecting vessels are divided into sections by valves. There are two lymphatic ducts also called lymphatic trunks , ductus thoracicus left lymphatic, or thoracic duct and ductus lymphaticus dexter right lymphatic duct , respectively located in the left and right thoracic chest region.
The two ducts connect the large lymphatic collecting vessels to the blood circulation via the subclavian veins which pump blood into the heart.
The ductus thoracicus connects to the left subclavian vein, while the ductus lymphaticus dexter connects to the right subclavian vein. The junction where the lymphatic ducts and subclavian veins meet is the only direct connection between the blood and lymph circulatory systems, and thus the only point where lymph can enter the blood circulation.
Thousands of lymph nodes occur in clusters along the lymphatic vessels. They are particularly abundant in the lower abdomen, neck and armpits. Each lymph node connects to several afferent lymphatic vessels on one side known as the cortex and lymph enters the node through these connections. Each node also connects to a smaller number of efferent lymphatic vessels on the other side known as the hilus and lymph exits the node through these connections.
Lymph nodes can be separated into a fibrous outer capsule and an inner cortex of soft tissue. The cortex is segmented by strands called trabeculae, which are extensions of the fibrous capsule. Lymph enters and travels through the lymph node cortex. The segments of the lymph node cortex support a dynamic changing population of lymphocytes. The lymphatic system as a whole supports an abundance of lymphocytes, with a collective weight of 1 kg in a 70 kg body.
Each node also contains static unchanging collections of macrophages. Structures called follicles within the lymph nodes support static populations of B-lymphocytes antibody-producing lymphocytes. Lymphoid tissue, also referred to as lymphoid nodules, is tissue that is dominated by the lymphocytes. A lymphoid nodule is usually about a millimetre in diameter, but as there is no capsule surrounding it, it is often hard to measure. Examples of these nodules include the gut-associated lymphatic tissue GALT cells as well as the tonsils.
The lining of other hollow organs also contain patches of lymphoid tissue. Lymphoid organs are characterised by abundant lymphocytes and connective structural tissues.
In addition to the lymph nodes they include:. While each of these organs and tissues fulfils protective immune functions which are related to the lymphatic system, the lymph nodes are unique amongst the lymphoid organs because they are the only organs with lymph filtering functions. The spleen is a blood-rich organ and the largest of the lymphoid organs.
It is usually purple in colour, and located in the upper-left section of the abdomen. The spleen is surrounded by the lining of the abdominal cavity on all sides except at the hilum, where the splenic artery and vein are located.
The spleen lies behind to the stomach and in front of the diaphragm, near the left kidney. It is covered by a fibrous capsule which is thickest at the hilum, where the splenic artery and vein connect and transport blood into and out of the spleen. The spleen is composed of areas of red pulp and white pulp. Most of the red pulp consists of loose tissues and blood capillaries.
The splenic white pulp is made of two types of lymphocytes; T lymphocyte infection detecting and B lymphocyte antibody producing. The thymus is a lymphoid organ located in the lower section of the throat, overlying the heart. It receives a rich supply of arterial blood from the large arteries which connect to it. The veins which drain blood from the thymus connect to larger veins in the chest area. The thymus is divided into two lobes which each have an outer cortex and an inner medulla.
The cortex sections contain T lymphocyte stem cells, while the medulla contains mature T lymphocytes which have migrated from the cortex. The thymus also contains hormone producing cells.
The lymphatic system commences development during embryogenesis the period from 4—8 weeks after fertilisation, in which the fertilised egg is called an embryo. First, several tiny sacs called lymph sacs form. These continue to grow to form lymphatic capillaries, vessels and nodes.
The vessels and clusters of nodes are visible by the 5 th week of embryo development the 7 th week of pregnancy which begins 2 weeks before fertilisation occurs. However, while the foetus remains in the womb the lymphatic system remains underdeveloped and does not function.
It is not until after childbirth when lymphocytes rapidly populate lymph nodes and immune functions commence. Before entering the lymphatic system, lymph is called interstitial fluid and consists mainly of a fluid called hyaluronan. This interstitial fluid plays an important role in giving shape and structure to the body organs, and in order to give each organ or body part the correct structure, the volume of interstitial fluid must remain constant.
If interstitial fluid accumulates, swelling occurs and the shape and structure of the organ or body part changes. So although the lymphatic system is constantly absorbing interstitial fluid from the interstitial spaces, there is always a constant volume of interstitial fluid in a given interstitial space except for example in times of inflammation and swelling.
The lymphatic system only absorbs fluid when new fluid is leaked into the space to replace that absorbed into the lymphatic system. Absorption of lymph into the lymphatic vessels plays an important role in maintaining the correct amount of fluid in the interstitial spaces to ensure that swelling does not occur. Lymph vessels collect the interstitial fluid and then return it to the bloodstream by emptying it into large veins in the upper chest, near the neck.
Lymph fluid enters the lymph nodes, where macrophages fight off foreign bodies like bacteria, removing them from the bloodstream. After these substances have been filtered out, the lymph fluid leaves the lymph nodes and returns to the veins, where it re-enters the bloodstream.
Lymphatic vessels and lymph nodes can be visualized by the process of lymphangiography. A radiopaque not transparent to x-rays contrast material is injected into a lymphatic vessel. The fluid is left in the system for 24 hours and the lymph nodes can then be observed by X-rays.
This technique is quite important in the treatment of neoplasms and other disorders of the lymphatic system. The technique is also used to locate lymph nodes for radiation therapy or for surgical removal. Please visit all of The Merck Manuals free online at www. The Lymphatic System The lymphatic system is an extensive drainage network that helps keep bodily fluid levels in balance and defends the body against infections. Functions of the Lymphatic System The lymphatic system has three functions: The removal of excess fluids from body tissues.
This process is crucial because water, proteins, and other substances are continuously leaking out of tiny blood capillaries into the surrounding body tissues. Absorption of fatty acids and subsequent transport of fat, chyle, to the circulatory system. Production of immune cells such as lymphocytes, monocytes, and antibody producing cells called plasma cells. Immunity and Spread of Infection The lymphatic system plays an integral role in the immune functions of the body.
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