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.
Lymph capillaries in spaces. Blind-ended lymphatic capillaries arise within interstitial spaces of cells near arterioles and venules.
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]
Lymph drainage flow; lymphatic duct anatomy.
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.