The lymphatic (lim-fat'ik) system consists of lymphatic vessels and the lymphoid organs. This system, which is closely associated with the cardiovascular system, has three main functions:
- Lymphatic vessels take up excess tissue fluid and return it to the bloodstream.
- Lymphatic capillaries absorb fats at the intestinal villi and transport them to the bloodstream
- The lymphatic system helps to defend the body against disease.
The lymphatic system: (1) collects excess tissue fluid, (2) absorbs fat molecules in the intestines, and (3) plays a major role in the body's defense against disease.
Lymphatic Vessels
Lymphatic vessels contain lymph. Lymph (lim£), which i excess tissue fluid, consists mostly of water and some plasma proteins that have leaked out of capillaries. Lymphatic vessels are quite extensive; every region of the body is supplied richly with lymphatic capillaries. The structure of the larger lymphatic vessels is similar to that of cardiovascular veins, including the presence of valves. Also, the movement of lymph within these vessels is dependent upon skeletal muscle contraction. When the muscles contract, lymph is squeezed past a valve that closes, preventing lymph from flowing backward.
The lymphatic system is a one-way system. The system begins with lymphatic capillaries that lie near blood capillaries. The lymphatic capillaries take up fluid that exited from, and was not reabsorbed by, the blood capillaries. Once tissue fluid enters the lymphatic vessels, it is called lymph. The lymphatic capillaries join to form lymphatic vessels that merge in the thoracic cavity before entering one of two ducts: the thoracic duct or the right lymphatic duct.
The thoracic duct is much larger than the right lymphatic duct. It serves the lower extremities, abdomen, left arm, and the left side of the head and neck. In the thorax, the left thoracic duct enters the left subclavian vein. The right lymphatic duct serves only the right arm and the right side of the head and the neck; it enters the right subclavian vein.
The lymphatic system is a one-way system. Lymph flows from a lymphatic capillary to ever-larger lymphatic vessels. and finally, to a lymphatic duct, which enters a subclavian vein.
Lymphoid Organs
The lymphoid organs are so-called because they contain lymphocytes. The lymphoid organs of special interest are the spleen, lymph nodes, thymus, and red bone marrow .
Spleen
The spleen is located in the upper left abdominal cavity, just beneath the diaphragm. Its construction is similar to that of a lymph node: The outer connective tissue divides the organ into sinus-containing lobules. In the spleen, however, the sinuses are filled with blood instead oflymph. Since the blood vessels of the spleen can expand, this organ serves as a blood reservoir and makes blood available in times of low pressure or when the body needs extra oxygen in the blood.
The lobules of the spleen contain red pulp and white pulp. Red pulp contains red blood cells, lymphocytes, and macrophages. White pulp forms nodules that contain only lymphocytes and macrophages. Both types of pulp help to purify the blood that passes through the spleen. If the spleen ruptures due to injury, it can be removed. Although the spleen's functions are duplicated by other organs, an individual whose spleen has been removed is slightly more susceptible to certain bacterial infections because of a loss of B lymphocytes.
The spleen is divided into sinus-containing lobules in which blood is cleansed by lymphocytes and macrophages.
Lymph Nodes
At certain points along lymphatic vessels are small (about 1-25 mm), ovoid or round structures called lymph nodes. A lymph node has a fibrous connective tissue capsule. Connective tissue also divides a node into nodules, each of which contains a sinus (open space) filled with many lymphocytes and macrophages. As lymph passes through the sinuses, it is purified of infectious organisms and any other debris.
While nodules usually occur within lymph nodes, they can also occur singly or in groups. The tonsils, which are partly encapsulated lymph nodules, are located in the pharynx. The pharyngeal tonsils, or adenoids, are situated in the nasopharynx. The palatine tonsils are located in the posterior lateral wall of the oropharynx. The lingual tonsils are at the base of the tongue in the oropharynx. Other nodules called Peyer's patches are located within the intestinal wall.
The lymph nodes occur in groups in certain regions of the body. For example, the inguinal nodes are in the groin, and the axillary nodes are in the armpits. The Medical Focus reading on this page examines illnesses associated with the lymph nodes.
Lymph nodes are divided into sinus-containing nodules in which the lymph is cleansed of infectious organisms and debris.
Thymus
The thymus is located along the trachea atop the heart and posterior to the sternum in the upper thoracic cavity. This gland varies in size, but it is larger in children than in adults and may eventually almost disappear. The thymus is divided into lobules by connective tissue. T lymphocytes ( or T cells), discussed later in the chapter, mature in these lobules. Those in the interior (medulla) are more mature than those in the exterior (cortex) of a lobule. Mature T lymphocytes have survived a critical test: If any show the ability to react ,vith "self" cells, they die. If they have the potential to attack a foreign celL they leave the thymus.
The thymus secretes thymosin, a hormone that is believed to be an inducing factor; that is, it causes pre-T cells to become T (for thymus) cells. Thymosin may also have other functions in immunity.
The thymus is divided into lobules where lymphocytes are produced.
Red Bone Marrow
Red bone marrow is the site of origination for all blood cells, including the white blood cells that function in immunity. In the adult, red bone marrow is present only in the bones of the skull, sternum, ribs, clavicle, pelvis, and spinal column, and in the ends of the femur and humerus.
Red bone marrow consists of a network of connective tissue fibers, called reticular fibers, produced by reticular cells. These cells, along with the cells that develop into blood cells, are packed around thin-walled venous sinuses. Differentiated blood cells enter the bloodstream at these sinuses.
Red bone marrow produces white blood cells necessary for the development of immunity.
