INTRODUCTION
• The lymphatic system forms a closed network of lymph vessels through which lymph circulates all over the body.
• The flow of lymphatic fluid is unidirectional, from tissue spaces to the blood.
• By circulation of lymphatic fluid, the body is protected against harmful agents.
• Interstitial fluid is produced by filtration of components of the blood plasma through the walls of blood capillary.
• This interstitial fluid is then termed as lymph as it moves into the lymphatic vessels.
• The lymphatic system consists of:
1) Lymph, a fluid
2) Lymphatic vessels that transport the lymph.
3) Lymph nodes through which lymph passes before retuming to the blood.
4) A number of structures and organs containing lymphatic tissue, e.g.
spleen and thymus
5) Red bone marrow, where stem cells develop into various types of blood cells, including lymphocytes.
6) Diffuse lymphoid tissue, e.g. tonsils.
Lymphatic Organs And Tissues
Immune system is a complex system comprising of a meshwork of Lymphatic organs, tissues, and cells. The organs and tissues involved in lymphatic system can be categorised as:
1) Primary Lymphatic organs
2) Secondary lymphatic organs.
PRIMARY LYMPHATIC ORGANS
The red bone marrow and the thymus form the primary lymphatic ore They serve as sites of production and maturation of lymphocytes.
They a discussed below:
1) Red Bone Marrow: The red bone marrow acts as the site of production
of blood cells. It is a soft, spongy, nutrient rich tissue present at the end of long bones (femur) or in flat bones (sternum, bones of skull etc).
• Neutrophils, basophils, eosinophils, monocytes, and lymphocytes are some of the white blood cells that are produced in the marrow.
• B and T lymphocytes are the two different types of cells into which lymphocytes differentiate. Maturation of B-lymphocytes also occurs in red bone marrow, while the T-lymphocytes mature in the thymus.
2) Thymus Gland: The thymus gland is found in the upper thoracic cavity.
• It is positioned posterior to the sternum and anterior to the ascending aorta.
• The thymus is divided into lobules by connective tissue.
• These lobules are rich in lymphocytes. The thymus gland produces thymic hormones, e.g., thymosin.
• In the absence of thymus gland, lymphocyte level in the blood is poor, an individual fails to discard foreign substances and the body's response against antigens is either lacking or becomes very weak.
SECONDARY LYMPHATIC ORGANS
• Spleen, lymph nodes, tonsils, Peyer's patches, and the appendix are the secondary lymphatic organs.
• The secondary lymphatic organs play significant role in the immune system because these form the sites where lymphocytes reach and bind with antigens. • This is then followed by the multiplication and activation of lymphocytes.
1) Spleen: Spleen is characteristically present in the Vertebrates and S closely related to the circulatory system.
• One of the major functions of spleen is the destruction of old RBCs.
• It forms the reservoir of blood which can be utilised during conditions like haemorrhagic shock.
• It s ductless gland, found in the upper left area of the abdominal cavity an is partly divided into compartments.
• Every compartment comprises of tissue identified as white pulp and red pulp.
• The white pulp is rich in lymphocytes, whereas the red pulp is known to assist the process of blood filtration,
•As the blood enters the spleen and passes through the sinuses for filtration, a series of events take place which involves the reaction of lymphocytes with pathogens, the consumption of debris by macrophages and the removal of old and worn-out red blood cells. In the absence of spleen, an individual becomes more vulnerable to infections.
2) Lymph Nodes: Lymph nodes are oval-shaped small nodes that are positioned along the lymphatic vessels.
• The structure of lymph nodes is like an internal honeycomb made up of connective tissues.
• It is rich in lymphocytes, which filter and destroy bacteria and viruses. A lymph node is further divided into compartments, comprising of a sinus and is packed with B-lymphocytes.
• As lymph passes through the sinuses, the macrophages filter and engulf the pathogens and debris.
• T-lymphocytes are also present in the sinuses.
• The purpose of T-lymphocytes is to fight against infections and attack cancerous cells.
• Except the dorsal cavity, the lymph nodes are present in almost every body cavity.
• During an infection, lymphocytes present in the lymph node proliferate rapidly and result to swelling of the lymph nodes.
3) Tonsils: The tonsils exist as a ringed structure around the pharynx, in three pairs. The pathogens and antigens entering the body through mouth or nose are initially interrupted by the tonsils.
4) Peyer's Patches: Peyer's patches are present in the walls of the intestine and the appendix, attached to the cecum of the large intestine. The pathogens that enter the body through the intestinal tract are interrupted by the Peyer's patches.
LYMPH :-
Lymph is a clear watery fluid. Except for plasma proteins, the composition of lymph is almost similar to the plasma and the interstitial fluid,
One of the major functions of the lymph is transportation of plasma proteins oozing out of the capillary beds, back to the bloodstream.
• In the process, it also carries large foreign bodies like bacteria and cell debris away from the damaged tissues.
• They are then filtered and destructed by the lymph nodes.
• Lymphocytes present in the lymph circulate in the lymphatic system through the different regions of the body.
• The lacteals are the lymphatic capillaries of the small intestine that absorb fat. Due to the presence of fat, the lymph (or chyle) appears milky in colour.
• The various functions of lymph include:
1) Nutritive: Nutrition and oxygen is supplied to low blood areas.
2) Drainage: Maintains constant volume and composition of tissue fluid
by removing excess fluids and metabolites 3) Transmission of Proteins: Lymph carries proteins to the through tissue spaces.
4) Absorption of Fats: Intestinal fats are absorbed through the lymphatics.
COMPOSITION OF LYMPH :-
FORMATION OF LYMPH :-
Factors participating in lymph formation are:
1) Capillary Pressure: It increases the rate of lymph formation.
2) Permeability of the Capillary Wall:
More capillary wall permeability means more tissue fluid formation resulting in more lymph.
~~ Factors that increase capillary permeability are:
i) Rise in Temperature:
• Increased temperature increases capillary permeability.
ii) Substances Acting Directly on the Capillary Wall:
• foreign proteins, histamine and extracts of strawberries, crayfish, mussels, leech, etc. cause injury to capillary which increases their permeability.
iii) Reduced Oxygen Supply:
Lack of oxygen results to higher limph flow that means more vessel permeability.
3) Substances Altering the Osmotic Pressure:
Decrease in blood colloidal osmotic pressure increases tissue fluid and lymph formation.
4) Increased Metabolic Activity of an Organ:
High activity area increases lymph flow at that site.
It is due to :-
i) Increased metabolite formation which elevates the osmotic pressure
of the tissue fluid,
ii) Increased capillary pressure and permeability due to local vasodilation.
iii) Relative anoxia, and
iv) Increased temperature.
LYMPH VESSELS :-
• These are tiny walled vessels.
• These are closed to one end.
• They are larger in diameter
• The main purpose is to drain excess intestial fluid from around the cell to venous circulation.
• Lymphatic capillaries combine together to form lymphatic vessels.
LYMPH NODES :-
• Lymph nodes are structures that are small and glandular in appearance.
• They are present all along the path of lymph vessels.
• They are also referred to as lymph glands or lymphatic nodes.
• They are organ of lymphatic system having variety of cells present within
the body, and functions to filter or trap foreign particles.
• White blood cells in them require oxygen to process. They have significance for immune system.
• There are many groups of lymph nodes: 1) Superficial and deep cervical lymph nodes are present in neck,
2) Axillary lymph nodes in axilla (armpit), and
3) Inguinal lymph nodes below the inguinal ligament on the front of the thigh.
STRUCTURE OF LYMPH NODE :-
• Every lymph node is composed of a mass of lymphatic tissue, which is enclosed within a dense connective tissue capsule.
• Lymph nodes are made of connective tissue, various lymphocytes, and other cells completing the ans in its network. Nodes are bean-shaped.
~~Three layers that constitute the structure of a lymph node are as follows:
1) Cortex:
• Cortex of lymph node consists of primary and secondary lymphoid follicles.
• Primary follicle develops first. When some antigens enter the body and reach the lymph nodes.
• the cells of primary follicle proliferate. The active proliferation of the cells occurs in a particular area of the follicle called the germinal centre.
• After proliferation of cells, the primary follicles become the secondary follicle. Cortex also contains some B lymphocytes, which are usually aggregated into the primary follicles. Macrophages are also found in the cortex.
2) Paracortex:
• The paracortex is the tissue is present in between the cortex and medulla and comprises of T lymphocytes.
3) Medulla:
• The medulla of a lymph node is densely occupied by the macrophages, the B cells and the antibody-producing plasma cells which have migrated from the cortex into the medulla.
• All these cells are embedded in a meshwork of reticular cells and fibres and
are vascularised (blood vessels of lymph nodes pass through medulla).
LOCATION OF LYMPH NODES :-
• The main sites of occurrence of lymph nodes include the elbow, axilla, knee and groin. Other sites include areas where many lymph vessels join together such as definite sites in the neck, thorax and the abdomen.
Functions of Lymph Nodes Lymph nodes perform the following functions: -
1) They form lymphocytes (B-lymphocytes and T-lymphocytes) by multiplying the pre-existing lymphocytes. Lymphocytes from lymph pass into the bloodstream.
2) Lymph destroys bacteria and other particulate matter by phagocytosis
using macrophages. Antigens stimulate lymphocytes proliferation. Thus, lymph nodes are involved in immune response to antigens.
3) Spread of cancer cells to body are temporarily halted while penetrating through the lymph vessels to the lymph nodes.
4) They are mechanical filters preventing poisonous the circulation material to enter into the circulation.
5) Y-globulin is produced by lymph nodes.
FUNCTION OF LYMPHATIC SYSTEM :-
Functions of Lymphatic System The functions of lymphatic system are:
1) Tissue Drainage:
• Approximately 21 litres of plasma fluid with dissolved substances and plasma protein travel through arterial end of the capillaries to tissues every 24 hour.
• About 3-4 litres of this fluid are drained by the lymphatic vessels and rest return to the bloodstream through capillary at its venous end.
• This mechanism is important otherwise tissues will saturate with water and blood volume decreases causing CVS failure.
2) Absorption in the Small Intestine: Central lacteals (lymphatic vessels) of the villi absorb fat and fat-soluble substances (vitamins A, D, E, and K).
3) Immunity:
• The lymphatic organs mainly involve production and maturation of lymphocytes and white blood cells provide immunity Bone marrow, the lymphatic tissue, produces lymphocytes.
• Lymphatic tissue is reticular connective tissue containing many lymphocytes.
Lymphocytes which participate in immune responses are:
i) B-cells: These cells participate in humoral immune response (opposite to cell-mediated immune response governed by T-cells) essential for adaptive immune system. B-cells, activated on interaction with antigens produce antibodies against antigens as Antigen Presenting Cells (APCS) that develops into memory B cells.
ii) T-cells: These cells T-cells belong to lymphocytes (a group of WBCs) responsible for cell-mediated immunity. The name indicates its organ of maturation i.e., thymus. Unlike B-cells and natural killer cells, they have a surface receptor called T-Cell Receptors (TCR). Many subsets of T cells with distinct functions have been discovered.
4) Transporting Dietary Lipids:
• Lipids and lipid-soluble vitamins, passing through lymphatic vessels are absorbed by GIT.
5) Defensive: The lymphocytes and monocytes are body's defence cells that remove bacteria from tissues.
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