Lymphatic system

Anatomy

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.

Start with the lymphatic system with our learning materials. 

Lymph

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.

When reading anatomy texts for hours, thoughts can easily fly to Narnia. But we know how to help! Check out our learning strategy on how to learn anatomy faster by being an effective reader.

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

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. 

Test your knowledge on the lymphatic system with this quiz.

Lymphatic vessels

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. 

Do you prefer learning by doing? Check out our free exam-prep guide to learning the lymphatic system, complete with quizzes and labeling activities. 

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.

The lymphatic ducts take the lymph into the right and left subclavian veins, which flow into the superior vena cava.

Lymph nodes

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.

Consolidate your knowledge about the lymphatic nodes with our study unit for this topic. 

Lymphocytes

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. 

Functions

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.