LYMPHATIC SYSTEM

The lymphatic system

LYMPHATIC SYSTEM:

A child falls and scrapes her knee. Is this likely to be a life-threatening injury? Probably not, even though the breaks in the skin have permitted the entry of thousands or even millions of bacteria. Those bacteria, however, will be quickly destroyed by the cells and organs of the lymphatic system. Although the lymphatic system may be considered part of the circulatory system, we will consider it separately because its functions are so different from those of the heart and blood vessels. Keep in mind, however, that all of these functions are interdependent.

Functions of the lymphatic system

1. The lymphatic system is responsible for returning tissue fluid to the blood

2. It is also responsible for protecting the body against foreign material.

Parts of the lymphatic system

The parts of the lymphatic system are the lymph, the system of lymph vessels, and lymphatic tissue, which includes lymph nodes and nodules, the spleen, and the thymus gland.

The Lymph

Lymph is the name for tissue fluid that enters lymph capillaries. As you may recall from, filtration in capillaries creates tissue fluid from blood plasma, most of which returns almost immediately to the blood in the capillaries by osmosis. Some tissue fluid, however, remains in interstitial spaces and must be returned to the blood by way of the lymphatic vessels. Without this return, blood volume and blood pressure would very soon decrease. The relationship of the Lymphatic vessels to the cardiovascular system is therefore important.

Importance of the Lymph

1. Fluid Balance

Lymph collects excess fluid, proteins, and waste products from tissues, helping to maintain proper fluid balance.

2.Immune Support

Lymph contains immune cells that help defend against infections and foreign substances, acting as a key component of the immune system.

3. Transport of Nutrients

Lymph vessels absorb and transport fats and fat-soluble vitamins from the digestive system to the bloodstream.

4. Waste Removal

Lymph nodes filter and remove harmful substances, toxins, and cellular waste from the lymph before it returns to the bloodstream.

5.Pathogen Defence

Lymph nodes are essential in trapping and destroying pathogens, such as bacteria and viruses, to prevent their spread.

6. Cell Communication

The lymphatic system facilitates communication between immune cells and tissues, helping coordinate immune responses.

Lymph Vessels

The system of lymph vessels begins as dead-end lymph capillaries found in most tissue spaces. Lymph capillaries are very permeable and collect tissue fluid and proteins.

Lacteals are specialized lymph capillaries in the villi of the small intestine; they absorb the fat-soluble end products of digestion, such as fatty acids and vitamins A, D, E, and K. Lymph capillaries unite to form larger lymph vessels, whose structure is very much like that of veins. There is no pump for lymph (as the heart is the pump for blood), but the lymph is kept moving within lymph vessels by the same mechanism that promote venous return. The smooth muscle layer of the larger lymph vessels constricts, and the one-way valves (just like those of veins) prevent back-flow of lymph.

Lymph vessels in the extremities, especially the legs, are compressed by the skeletal muscles that surround them; this is the skeletal muscle pump. The respiratory pump alternately expands and compresses the lymph vessels in the chest cavity and keeps the lymph moving.

Where is the lymph going? Back to the blood to become plasma again.

Cisterna Chyli and Thoracic Duct

The lymph vessels from the lower body unite in front of the lumbar vertebrae to form a vessel called the cisterna chyli, which continues upward in front of the backbone as the thoracic duct. Lymph vessels from the upper left quadrant of the body join the thoracic duct, which empties lymph into the left subclavian vein.

Right Lymphatic Duct

Lymph vessels from the upper right quadrant of the body unite to form the right lymphatic duct, which empties lymph into the right subclavian vein. Flaps in both subclavian veins permit the entry of lymph but prevent blood from flowing into the lymph vessels.

Lymphatic tissue consists mainly of lymphocytes in a mesh-like framework of connective tissue. Recall that most lymphocytes are produced from stem cells in the red bone marrow, and then migrate to the lymph nodes and nodules, to the spleen, and to the thymus. In these structures, lymphocytes become activated and proliferate in response to infection (this is a function of all lymphatic tissue). The thymus has stem cells that produce a significant portion of the T-Iymphocytes.

Lymph Nodes and Nodules

Lymph nodes and nodules are masses of lymphatic tissue. Nodes and nodules differ with respect to size and location. Nodes are usually larger, 10 to 20 mm in length, and are encapsulated; nodules range from a fraction of a millimeter to several millimeters in length and do not have capsules.

Lymph Nodes

Lymph nodes are found in groups along the pathways of lymph vessels, and lymph flows through these nodes on its way to the subclavian veins. Lymph enters a node through several afferent lymph vessels and leaves through one or two efferent vessels. As lymph passes through a lymph node, bacteria and other foreign materials are phagocytized by fixed (stationary) macrophages. Plasma cells develop from lymphocytes exposed to pathogens in the lymph and produce antibodies. These antibodies will eventually reach the blood and circulate throughout the body.

Special lymph nodes

There are many groups of lymph nodes along all the lymph vessels throughout the body, but three paired groups deserve mention because of their strategic locations. These are the head, cervical, axillary, and inguinal lymph nodes. Notice that these are at the junctions of the head and extremities with the trunk of the body. Breaks in the skin, with entry of pathogens, are much more likely to occur in the-arms or legs or head rather than in the trunk. If these Pathogens get to the lymph, they will be destroyed by the lymph nodes before they get to the trunk, before the lymph is returned to the blood in the subclavian veins. You may be familiar With the expression “swollen glands,” as when a child has a strep throat (an inflammation of the pharynx caused by Streptococcus bacteria). These “glands” are the cervical lymph nodes that have enlarged as their macrophages attempt to destroy the bacteria in the lymph from the pharynx Lymph nodules.

Lymph nodules are small masses of lymphatic tissue found just beneath the epithelium of all mucous membranes. The body systems lined with mucous membranes are those that have openings to the environment: the respiratory, digestive, urinary, and reproductive tracts.

You can probably see that these are also strategic locations for lymph nodules, because any natural body opening is a possible portal of entry for pathogens. For example, if bacteria in inhaled air get through the epithelium of the trachea, lymph nodules with their macrophages are in position to destroy these bacteria before they get to the blood.

Special Lymph Nodules

Some of the lymph nodules have specific names. Those of the small intestine are called Peyer’s patches, and those of the pharynx are called tonsils. The palatine tonsils are on the lateral walls of the pharynx, the adenoid (pharyngeal tonsil) is on the posterior wall, and the lingual tonsils are on the base of the tongue. The tonsils, therefore, form a ring of lymphatic tissue around the pharynx, which is a common pathway for food and air and for the pathogens they contain. A tonsillectomy is the surgical removal of the palatine tonsils and the adenoid and may be performed if the tonsils are chronically inflamed and swollen, as may happen in children. As mentioned earlier, the body has redundant structures to help ensure survival if one structure is lost or seriously impaired. Thus, there are many other lymph nodules in the pharynx to take over the function of the surgically removed tonsils.

SPLEEN

The spleen is located in the upper left quadrant of the abdominal cavity, just below the diaphragm, behind the stomach. The lower rib cage protects the spleen from physical trauma.

In the fetus, the spleen produces red blood cells, a function assumed by the red bone marrow after birth. After birth the spleen is very much like a large lymph node, except that its functions affect the blood that flows through it rather than lymph. The functions of the spleen after birth are:

1. Contains plasma cells that produce antibodies to foreign antigens.

2. Contains fixed macrophages (RE cells) that phagocytize pathogens or other foreign material in the blood. The macrophages of the spleen also phagocytize old red blood cells and form bilirubin. By way of portal circulation, the bilirubin is sent to the liver for excretion in bile.

3. Stores platelets and destroys them when they are no longer useful.

The spleen is not considered a vital organ, because other organs compensate for its functions if the spleen must be removed. The liver and red bone marrow will remove old red blood cells and platelets from circulation. The many lymph nodes and nodules will phagocytize pathogens (as will the liver) and have lymphocytes to be activated and plasma cells to produce antibodies. Despite this redundancy, a person without a spleen is somewhat more susceptible to certain bacterial infections such as pneumonia and meningitis.

THYMUS

The thymus is located inferior to the thyroid gland. In the foetus and infant, the thymus s large and extends under the sternum. With increasing age, the thymus shrinks, and relatively little thymus tissue is found in adults, though it is still active. The stem cells of the thymus produce

T lymphocytes or T cells. Thymic hormones are necessary for what may be called “immunological competence.” To be competent means to be able to do something well. The thymic hormones enable the T cells to participate in the recognition of foreign antigens and to provide immunity. This capability of T cells is established early in life and then is perpetuated by the lymphocytes themselves. The newborn’s immune system is not yet fully mature, and infants are more susceptible to certain infections than are older children and adults. Usually by the age of 2 years, the immune system matures and becomes fully functional. This is why some vaccines, such as the measles vaccine, are not recommended for infants younger than 15 to 18 months of age. Their immune systems are not mature enough to respond strongly to the vaccine, and the protection provided by the vaccine may be incomplete.

FUNCTIONS OF THE THYMUS

1. T-cell Maturation

The thymus is the primary site where T-cells (a type of white blood cell) mature and differentiate into various subsets, acquiring specific immune functions.

2. Immune Cell Production

It produces and releases immature T-cells into the bloodstream, where they continue to mature in other lymphoid tissues.

3.Central Tolerance

The thymus plays a crucial role in central tolerance, a process that eliminates self-reactive T-cells to prevent autoimmune reactions.

4.Educating T-cells

During T-cell maturation, the thymus educates T-cells to recognize foreign antigens while ensuring tolerance to self-antigens.

5. Immune Surveillance

Mature T-cells generated in the thymus actively patrol the body, identifying and eliminating infected or abnormal cells, providing immune surveillance.

6. Cell-Mediated Immunity

The thymus contributes to cell-mediated immunity, which involves T-cells directly targeting and destroying infected or cancerous cells.

7.Adaptive Immune Responses

T-cells produced by the thymus are a crucial part of the adaptive immune response, enabling the body to mount specific defenses against pathogens.

8.Immuno-regulation

The thymus is involved in regulating immune responses, ensuring the balance between immune activation and suppression.

9.Aging and Immunity

The thymus’s function declines with age, which can impact immune responses and increase susceptibility to infections.

10.Thymic Hormone Production

The thymus produces thymic hormones, such as thymosin, which can influence T-cell development and immune system function.