The lymphatic system parallels the cardiovascular system (see the images below). The lymphatic system is unique, in that it is a 1-way system that returns lymph fluid via vessels to the cardiovascular system for eventual elimination of toxic byproducts by end organs, such as the kidney, liver, colon, skin, and lungs.
The lymphatic system consists of the following (see Table 1 below):
Fluid, known as lymph
Vessels that transport lymph
Organs that contain lymphoid tissue (eg, lymph nodes, spleen, and thymus)
Table 1. Key Components of Lymphatic System (Open Table in a new window)
|Lymph||Contains nutrients, oxygen, hormones, and fatty acids, as well as toxins and cellular waste products, that are transported to and from cellular tissues|
|Lymphatic vessels||Transport lymph from peripheral tissues to the veins of the cardiovascular system|
|Lymph nodes||Monitors the composition of lymph, the location of pathogen engulfment and eradication, the immunologic response, and the regulation site|
|Spleen||Monitors the composition of blood components, the location of pathogen engulfment and eradication, the immunologic response, and the regulation site|
|Thymus||Serves as the site of T-lymphocyte maturation, development, and control|
The lymphatic system’s main functions are as follows:
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
Lymph is a fluid derived from blood plasma. It is pushed out through the capillary wall by pressure exerted by the heart or by osmotic pressure at the cellular level. Lymph contains nutrients, oxygen, and hormones, as well as toxins and cellular waste products generated by the cells. As the interstitial fluid accumulates, it is picked up and removed by lymphatic vessels that pass through lymph nodes, which return the fluid to the venous system. As the lymph passes through the lymph nodes, lymphocytes and monocytes enter it.
At the level of the gastrointestinal (GI) tract, lymph has a milky consistency that is attributable to fatty acids, glycerol, and rich fat content. Lacteals are lymph vessels that transport intestinal fat and are localized to the GI tract. [1, 5, 3]
Lymphatic capillaries are blind-ended tubes with thin endothelial walls (only a single cell in thickness). They are arranged in an overlapping pattern, so that pressure from the surrounding capillary forces at these cells allows fluid to enter the capillary (see the image below). The lymphatic capillaries coalesce to form larger meshlike networks of tubes that are located deeper in the body; these are known as lymphatic vessels.
The lymphatic vessels grow progressively larger and form 2 lymphatic ducts: the right lymphatic duct, which drains the upper right quadrant, and the thoracic duct, which drains the remaining lymphatic tributaries. Like veins, lymphatic vessels have 1-way valves to prevent any backflow (see the image below). The pressure gradients that move lymph through the vessels come from skeletal muscle action, smooth muscle contraction within the smooth muscle wall, and respiratory movement. [1, 6, 2, 5, 4, 7]
Lymph nodes are bean-shaped structures that are widely distributed throughout the lymphatic pathway, providing a filtration mechanism for the lymph before it rejoins the blood stream. The average human body contains approximately 600-700 of them, predominantly concentrated in the neck, axillae, groin, thoracic mediastinum, and mesenteries of the GI tract. Lymph nodes constitute a main line of defense by hosting 2 types of immunoprotective cell lines, T lymphocytes and B lymphocytes.
Lymph nodes have 2 distinct regions, the cortex and the medulla. The cortex contains follicles, which are collections of lymphocytes. At the center of the follicles is an area called germinal centers that predominantly host B-lymphocytes while the remaining cells of the cortex are T-lymphocytes. Vessels entering the lymph nodes are called afferent lymphatic vessels and, likewise, those exiting are called efferent lymphatic vessels (see the image below).
Extending from the collagenous capsule inward throughout the lymph node are connective tissue trabeculae that incompletely divide the space into compartments. Deep in the node, in the medullary portion, the trabeculae divide repeatedly and blend into the connective tissue of the hilum of the node. Thus the capsule, the trabeculae, and the hilum make up the framework of the node. Within this framework, a delicate arrangement of connective tissue forms the lymph sinuses, within which lymph and free lymphoid elements circulate.
A subcapsular or marginal sinus exists between the capsule and the cortex of the lymph node. Lymph passes from the subcapsular sinus into the cortical sinus toward the medulla of the lymph node. Medullary sinuses represent a broad network of lymph channels that drain toward the hilum of the node; from there, lymph is collected into several efferent vessels that run to other lymph nodes and eventually drain into their respective lymphatic ducts (see the image below). [1, 6]
The thymus is a bilobed lymphoid organ located in the superior mediastinum of the thorax, posterior to the sternum. After puberty, it begins to decrease in size; it is small and fatty in adults after degeneration.
The primary function of the thymus is the processing and maturation of T lymphocytes. While in the thymus, T lymphocytes do not respond to pathogens and foreign organisms. After maturation, they enter the blood and go to other lymphatic organs, where they help provide defense. Structurally, the thymus is similar to the spleen and lymph nodes, with numerous lobules and cortical and medullary elements. It also produces thymosin, a hormone that helps stimulate maturation of T lymphocytes in other lymphatic organs. [2, 5, 3, 4]
The spleen, the largest lymphatic organ, is a convex lymphoid structure located below the diaphragm and behind the stomach. It is surrounded by a connective tissue capsule that extends inward to divide the organ into lobules consisting of cells, small blood vessels, and 2 types of tissue known as red and white pulp. Red pulp consists of venous sinuses filled with blood and cords of lymphocytes and macrophages; white pulp is lymphatic tissue consisting of lymphocytes around the arteries. Lymphocytes are densely packed within the cortex of the spleen.
The spleen filters blood in much the same way that lymph nodes filter lymph. Lymphocytes in the spleen react to pathogens in the blood and attempt to destroy them. Macrophages then engulf and phagocytose damaged cells and cellular debris. The spleen, along with the liver, eradicates damaged and old erythrocytes from the blood circulation. Like other lymphatic tissue, it produces lymphocytes in an immunologic response to offending pathogens. [5, 3, 4]
Therefore, the spleen conducts several important functions, as follows:
It serves as a reservoir of lymphocytes for the body
It filters blood
It plays an important role in red blood cell and iron metabolism through macrophage phagocytosis of old and damaged red blood cells
It recycles iron by sending it to the liver
It serves as a storage reservoir for blood
It contains T lymphocytes and B lymphocytes for immunologic response
Tonsils are aggregates of lymph node tissue located under the epithelial lining of the oral and pharyngeal areas. The main areas are the palatine tonsils (on the sides of the oropharynx), the pharyngeal tonsils (on the roof of the nasopharynx; also known as adenoids), and the lingual tonsils (on the base of the posterior surface of the tongue).
Because these tonsils are so closely related to the oral and pharyngeal airways, they may interfere with breathing when they become enlarged. The predominance of lymphocytes and macrophages in these tonsillar tissues offers protection against harmful pathogens and substances that may enter through the oral cavity or airway. [1, 2]
Diseases of the lymphatic system include lymphedema, lymphoma, lymphadenopathy, lymphadenitis, filariasis, splenomegaly, and tonsillitis.
Lymphedema results when the lymphatic system cannot adequately drain lymph, resulting in an accumulation of fluid that causes swelling. It may be either primary or secondary.
Primary lymphedema is an inherited condition that occurs as a result of impaired or missing lymphatic vessels; it may be present at birth, may develop with the onset of puberty, or may occur in adulthood, with no apparent causes
Secondary lymphedema is basically acquired regional lymphatic insufficiency, which may occur as a consequence of any trauma, infection, or surgical procedure that disrupts the lymphatic vessels or results in the loss of lymph nodes 
Lymphoma is a medical term used for a group of cancers that originate in the lymphatic system. Lymphomas usually begin with malignant transformation of the lymphocytes in lymph nodes or bunches of lymphatic tissue in organs like the stomach or intestines. Hodgkin lymphoma and non-Hodgkin lymphoma are the 2 major categories of lymphoma, characterized by enlargement of lymph nodes, usually present in the neck. Symptoms of lymphoma include chronic fatigue, weak immune function, weight loss, and night sweats. 
Lymphadenopathy is a lymphatic disorder in which the lymph nodes become swollen or enlarged as a consequence of an infection. For example, swollen lymph nodes in the neck may occur as a result of a throat infection or sinus infection. 
Lymphadenitis is an inflammation of the lymph node that is due to a bacterial infection of the tissue in the node, which causes swelling, reddening, and tenderness of the skin overlying the lymph node. 
Splenomegaly, or enlarged spleen, is a lymphatic system disorder that develops as a result of a viral infection, such as mononucleosis.
Tonsillitis is caused by an infection of the tonsils (the lymphoid tissues present in the back of the oral cavity). The tonsils help filter out bacteria; when infected, they become swollen and inflamed, leading to sore throat, fever, and difficulty and pain while swallowing.