The lymphatic system is a system of specialized vessels and organs whose main function is to return the lymph from the tissues back into the bloodstream.
Lymphatic system is considered as a part of both the circulatory and immune systems, as well as a usually neglected part of students' books. The functions of the lymphatic system complement the bloodstream functions, as it regulates the balance of fluids in the body and filters the pathogens from the blood.
|Definition||The lymphatic system is a network of vessels and organs that regulates the amount of fluid in the human body and defends it against infections.|
|Structure||Lymphatic capillaries; lymphatic vessels, ducts and tracts; primary and secondary lymphoid organs,|
|Function||Fluid regulation; immune surveillance; transport of large molecules|
|Clinical relations||Cancer spreading|
This article will discuss the anatomy and functions of the lymphatic system.
- Lymphatic capillaries
- Lymphatic vessels
- Lymph nodes
- Lymphoid organs and immunity
- Cancer spreading
The lymphatic system begins with the lymphatic capillary meshwork that collects the excessive fluid from the tissues.
The lymph travels from the tissues through larger lymph vessels until it reaches its destination point; the bloodstream. On the way, it traverses lymphoid organs filled with immune cells that monitor if there are any pathogens in the incoming lymph.
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Lymph is a clear, yellowish fluid present in most tissues of the body. It is created as a result of the filtration of the plasma. The plasma from the blood diffuses through the porous capillary wall into the tissues to deliver nutrients. After feeding the hungry cells on the periphery, the majority of fluid gets reabsorbed back into the blood vessels, while around 10% of the fluid stays in the tissue. That amount of residual fluid in the tissues is called the interstitial fluid. When the interstitial fluid gets absorbed into the lymphatic capillaries it becomes the lymph.
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Similar to blood plasma, the lymph is composed mainly of water. The other components are proteins, lipids, glucose, ions, and cells. Depending on where the lymph is produced, the composition of lymph can vary (e.g. lymph produced in the gastrointestinal system is rich in fats). A body of a healthy individual produces an average of 2 liters of lymph per day, but this amount can vary greatly in pathological conditions.
Lymphatic capillaries are the smallest lymphatic vessels that collect the interstitial fluid from the tissues. They are organized in networks called lymphatic plexuses. Plexuses converge to make larger lymphatic vessels that carry the lymph away from the tissues and into the bloodstream.
There are also special types of lymphatic capillaries called lacteals. These capillaries absorb nutrients from the small intestine.
The lymphatic vessels are divided into two large groups; superficial and deep lymphatic vessels. The superficial vessels are located in the subcutaneous layer of the skin where they collect the lymph from the superficial structures of the body. They tend to follow the drainage of the venous system and in the end, drain into deep lymphatic vessels. The deep lymphatic vessels carry lymph from internal organs. In contrast to the superficial vessels, the deep vessels are accompanied by the arteries. These arteries lean onto the walls of the deep lymphatic vessels, putting pressure upon them and helping the flow of the lymph.
Along the way, both superficial and deep lymphatic vessels go through lymph nodes that monitor the content of the lymph. Lymphatic vessels that carry lymph towards the lymph node are known as afferent, whereas the vessels that carry lymph away from the lymph node are called efferent lymphatic vessels.
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The efferent vessels empty into the lymphatic trunks. The lymphatic trunks are named according to the region of the body that they drain the lymph from. There are four pairs of trunks: lumbar, bronchomediastinal, subclavian and jugular. There is also one unpaired intestinal lymph trunk, that drains lymph from the majority of organs of the gastrointestinal tract. The duct opens in the cisterna chyli which is the dilated origin of the thoracic duct.
The lymphatic trunks then converge into the two lymphatic ducts; the right lymph duct and thoracic duct.
- The right lymphatic duct collects lymph from the right upper limb and the right side of the head and chest.
- The thoracic duct is a larger vessel and collects lymph from the rest of the body.
Lymphatic vessels vs. blood vessels
The lymphatic vessels should not be confused with blood vessels. First of all, the lymphatic system is a one-way street starting blindly in the tissues and opening into the circulatory system on the other end. On the other hand, the venous and arterial vessels of the circulatory system vessels are connected by capillary networks and thus the blood flows in circles. The lymphatic system doesn’t have a pump that can regulate the pressure of the flow of the lymph like the circulatory system has (the heart). Instead, the lymph flows thanks to the movements of the body, pulsation of the arteries and contractions of skeletal muscles. The lymphatic vessels have valves that prevent the lymph flowing backwards.
Lymphatic vessels are located throughout the whole body but note that some tissues and organs are lacking the lymphatic vessels (e.g. epidermis, cartilage, bone marrow, the structures of the eye). For a long time, it has been believed that the central nervous system doesn't contain lymph vessels. Now there is convincing evidence that the lymphatics do exist in some parts of the central nervous system.
The lymph nodes are secondary lymphoid organs distributed throughout the whole body, grouped according to the body regions they are in (e.g. axillary, pelvic, mediastinal lymph nodes). An adult human has an average of 450 lymph nodes, most of which are located in the abdomen.
The lymph nodes house lymphocytes and other immune cells (e.g. macrophages, plasmocytes, dendrocytes). Thanks to the many immune cells found within them, the lymph nodes serve as a filtration point for the lymph that travels towards the venous system. In case the immunocytes detect a foreign particle in the lymph (e.g. microorganism), they will start the immune response to prevent the harming particle from disseminating throughout the body.
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The lymphocytes are one of the body's main immune cells. They arise from the stem cells in the primary lymphoid organs and belong to the part of the immune system called the acquired immunity. After maturation, the lymphocytes are distributed mainly in the secondary lymphoid organs.
According to their histology and functional characteristics, the lymphocytes are divided into three major groups; B lymphocytes, T lymphocytes and natural killer (NK) cells. Their main role is to establish a specific immune response to foreign particles (antigens). B lymphocytes destroy the antigens indirectly, by producing antigen-specific antibodies that attach to antigens and mark them for destruction. On the other hand, T lymphocytes and NK cells directly kill cells that are infected by viruses or become cancerous.
Lymphoid organs and immunity
According to their function and structure, the lymphoid organs are divided into two groups; primary and secondary lymphoid organs.
Primary lymphoid organs
The primary lymphoid organs serve as a nursery for the formation and development of the lymphocytes. There are two primary lymphoid organs; the bone marrow and the thymus. The bone marrow contains the stem cells from which the lymphocytes originate. B cells fully develop in the bone marrow. On the other hand, the T cells arise from the stem cells in the bone marrow but then travel to the thymus to complete their differentiation.
This process of development of both types of lymphocytes is called an antigen-native development. After this initial development, the lymphocytes enter the bloodstream which carries them throughout the body and disperses them in the connective tissues and the secondary lymphoid organs.
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Secondary lymphoid organs
The secondary lymphoid organs are the spleen, tonsils, vermiform appendix, lymph nodes, and specialized lymphoid tissue of the mucosae (MALT). When their maturation process in the primary organs ends, the lymphocytes relocate into the specific areas of these secondary lymphoid organs. There, they meet the antigens for the first time and undergo final maturation process called the antigen-dependent activation. This process prepares them for the battle against specific antigens.
Check out our learning materials about the spleen and test your knowledge with our integrated quizzes.
The lymphatic system has several crucial functions for maintaining body homeostasis which include: maintaining the body's fluid balance, transportation of large molecules and immune surveillance.
The fluid balance is maintained by draining the extra fluid that remains after the exchange of blood and nutrients between the tissues and capillaries. If not regularly drained, this amount of fluid can accumulate and cause swelling (edemas). Lymph also carries the molecules that are too large to diffuse through the capillary wall (e.g. proteins or lipids). This is why the small intestine has a vast lymphatic drainage, as it is the site where the lipids and proteins are absorbed from during food digestion. The lymphatic organs house numerous immune system cells which surveil the content of the lymph as it flows toward the venous system. If a foreign particle is detected, the immune cells start an immune response to destroy the pathogen and prevent the infection and damage.
Let’s recap the lymphatic system functions:
- The best-known function of the lymphatic system is its role in body fluid balance regulation by returning the excess fluid and proteins into the venous system.
- The lymph helps large molecules that cannot diffuse through the capillary wall to enter the blood, like proteins or lipids.
- The lymph system also has a major role in immune surveillance and fighting pathogens found in the body.
In clinical world the process of cancer spreading is called metastasis. While the lymphatic tissue can be a primary site for cancer (e.g. lymphoma), it is more commonly a pathway for a metastatic process. The other major pathway for cancer spreading is through the bloodstream.
The lymphatic vessels pick up the cancer cells when they penetrate the basement membrane of the altered tissue and relocate in the underlying connective tissue. From there, cancer cells get into the lymphatic capillaries and lymph stream which carries them into the closest regional lymph node. The first lymph node that drains the cancer is called the sentinel lymph node (guardian lymph node).
Pathological examination of the sentinel lymph node is very important for prognosis and staging of cancer. If the tumor cells are found only in the sentinel lymph node, i.e. regional lymph node, it is an indication that the tumor is in an early stage. On the other hand, if the tumor has affected the lymph nodes far away from the initial tumor, it may indicate that the tumor is in its later stage.