Lymphatic System Structure and Function

The lymphatic system is a part of the circulatory system [also called subsystem of the circulatory system] that is formed by a network of network of vessels [other than arteries and veins], tissues, and organs.

The main function of the system is

• Maintainfluid balance in the body by collecting excess fluid depositing them in the bloodstream.
• Provides immunity by transporting immune cells
• Restoration of excess interstitial fluid and proteins to the blood
• Absorption of fats and fat-soluble vitamins from the digestive system and transport of these elements to the venous circulation
• Defense against invading organisms

Transports particulate matter and other substances from tissues to the bloodstream. The lymphatic system thus participates in the elimination of toxic byproducts by end organs, such as the kidney, liver, colon, skin, and lungs.

Structure of the Lymphatic System

The lymphatic system consists of

• Fluid called lymph
• Vessels that transport lymph
• Organs containing lymphoid tissue (eg, lymph nodes, spleen, and thymus)

Lymphatic vessels from different parts converge to form one of two large vessels called lymphatic trunks, which are connected to veins at the base of the neck.

One of these trunks, the right lymphatic duct, drains the upper right portion of the body, returning lymph to the bloodstream via the right subclavian vein.

The other trunk, the thoracic duct, drains the rest of the body into the left subclavian vein. Lymph is transported along the system of vessels by muscle contractions.

and valves prevent lymph from flowing backward. The lymphatic vessels are punctuated at intervals by small masses of lymph tissue, called lymph nodes, that remove foreign materials such as infectious microorganisms from the lymph filtering through them.

Lymph

Lymph is a fluid derived from blood plasma.

As the blood is circulated in the body, some plasma leaks into the tissues through the thin walls of the capillaries, due to pressure exerted by the heart.

Osmotic pressure at the cellular level is also responsible.

This fluid is called the interstitial or extracellular fluid.

It contains nutrients, oxygen, and hormones, as well as toxins and cellular waste products generated by the cells.

This fluid is removed by the lymphatic system.

Thus the lymphatic system serves to prevent fluid imbalance.

This return starts with fluid entering tiny lymphatic capillaries that are present in almost every tissue of the body.

[Epidermis, mucous membranes, bone marrow, and the central nervous system, do not have lymphatic capillaries]

The extracellular fluid thus entering the lymphatic capillaries is known as lymph. From lymphatic capillaries, it enters the larger vessels called the lymphatics, the vessels that pass through lymph nodes and return the fluid to the venous system.

As the lymph passes through the lymph nodes, lymphocytes and monocytes enter it to enter into the bloodstream for distribution.

Lymph in the gastrointestinal tract has a milky consistency due to dissolved fats. This lymph is known as chyle.

Lacteals are lymph vessels that transport intestinal fat. They are found in the gastrointestinal tract only.

Lymphatic vessels

Lymphatic capillaries are the smallest lymph vessels. These are blind-ended tubes with thin endothelium consisting of one cell thickness only.

The lymphatic capillaries coalesce to form lymphatic vessels. Lymphatic vessels are larger meshlike networks of tubes and are located deeper.

These  lymphatic vessels grow progressively larger and form two lymphatic ducts

• Right lymphatic duct
  • Drains the upper right quadrant into the right subclavian vein
• Thoracic duct, which drains the lymphatic tributaries from rest of the body into the left subclavian vein.

Lymphatic vessels have 1-way valves similar to veins which prevent backflow.

Movement of lymph is caused by a gradient created muscle contraction [skeletal muscles/smooth muscles and respiration.

Organs containing Lymphoid Tissue

Lymph nodes

Lymph nodes are bean-shaped structures present distributed throughout the lymphatic pathway.

There are about 600-700 lymph nodes in the body. They are mainly present in the neck, axillae, mediastinum, gastrointestinal mesenteries groin.

These act as filters for the lymph before it is sent to the bloodstream.

The lymph nodes also contain T lymphocytes and B lymphocytes and act as the first barrier of defense.

Lymph nodes have 2 distinct regions, the cortex, and the medulla. The cortex contains follicles, which are collections of lymphocytes.

Vessels entering the lymph nodes are called afferent lymphatic vessels and, likewise, those exiting are called efferent lymphatic vessels

Within the lymph node, there is a unique arrangement of sinuses where the lymph flows.

The lymph is collected into several efferent vessels that run to other lymph nodes and eventually drain into their respective lymphatic ducts.

Thymus

It is located in the superior mediastinum, posterior to the sternum. It is a bilobed structure containing lymphoid tissue decreases in size after puberty to become small and degenerated [fat filled] in adults.

The thymus is responsible for processing and maturation of T lymphocytes which then enter the blood.

The structure of thymus is similar to lymph node and spleen.

It also produces a hormone called thymosin. This hormone stimulates maturation of T lymphocytes in other lymphatic organs. 

Spleen

The spleen is the largest lymphatic organ. It is located below the diaphragm and behind the stomach.

Its capsule extends inward to form a retinaculum which divides the organ into lobules containing small blood vessels, and 2 types of tissue known as red and white pulp.

Red pulp contains venous sinuses filled with blood and collection of cells [lymphocytes, blood cells, and macrophages] in the interstices of the reticulum, in between the sinusoids.

White pulp is lymphatic tissue consisting of lymphocytes around the arteries.

The spleen conducts several important functions, as follows:

• Blood

• • Filters blood
  • Storage reservoir for blood
• Red cell and iron metabolism
  • Phagocytosis of old and damaged red blood cells
  • Recycles iron by sending it to the liver
• Role in immunity
  • Stores lymphocytes for the body
  • Lymphocyte in the spleen initiate an immunologic response

Tonsils

Tonsils are aggregates of lymph node tissue located under the epithelial lining of the oral and pharyngeal areas. They are

• Palatine tonsils- sides of the oropharynx
• Pharyngeal tonsils or adenoids- roof of the nasopharynx
• Lingual tonsils-  base of the posterior surface of the tongue).

Tonsils contain lymphocytes and macrophages and protect against harmful pathogens entering through airway or oral cavity

Clinical Significance

Following are significant diseases of the lymphatic system

Lymphedema

Lymphedema is swelling that occurs when the lymphatic system cannot adequately drain lymph.

Primary lymphedema is hereditary.

Secondary lymphedema can occur as a result of trauma, infection, or surgery resulting in disruption of vessels.

Lymphoma

Lymphoma is a term for a group of malignancies that originate in the lymphatic system. Hodgkin lymphoma and non-Hodgkin lymphoma are the two major categories resulting in multiple lymph node swellings.

Lymphadenopathy

Lymph nodes enlargement due to infection, inflammation or malignancy.

Lymphadenitis

Inflammation of the lymph node.

Filariasis

Infection by filaria parasite resulting in lymphatic insufficiency.

Splenomegaly

An enlarged spleen can occur in many viral or parasitic infections such as mononucleosis, malaria etc.

Tonsillitis

It is caused by an infection of the tonsils resulting in redness and swelling.

References

• Brotons ML, Bolca C, Fréchette E, Deslauriers J. Anatomy and physiology of the thoracic lymphatic system. Thorac Surg Clin. 2012 May. 22(2):139-53.