Lymphatic system: A network of tissue, organs, and vessels that help to maintain the body’s fluid balance, cleanse the body fluid of foreign matter and provide immune cells for defence.

Lymph, the fluid that the system collects from the interstitial spaces of the tissue and returns to the bloodstream.

Lymphatic plexuses, networks of small lymphatic vessels, lymphatic capillaries, that originate in the extracellular spaces of most tissues.

Lymphatic vessels (lymphatics), a nearly body wide network of thin-walled vessels with abundant valves, originating from lymphatic plexuses along which lymph nodes are located.

Lymphatic vessels occur almost everywhere blood capillaries are found, except, for example, teeth, bone, bone marrow, and the entire central nervous system (excess fluid here drains into the cerebrospinal fluid).

Lymph nodes, small masses of lymphatic tissue through which lymph is filtered on its way to the venous system.

Lymphocytes, circulating cells of the immune system that react against foreign materials.

Lymphoid tissue, sites that produce lymphocytes, such as that found in the walls of the digestive tract; in the spleen, thymus, and lymph nodes; and in myeloid tissue in red bone marrow.

Lymphatic vessels resemble veins in structure with these exceptions.

Lymphatics vessels have thinner walls.

Lymphatics vessels contain more valves.

Lymphatics vessels contain lymph nodes located at certain intervals along their course.

The lymphoid tissue is primarily involved in immune responses, and consists of lymphocytes and other white blood cells enmeshed in connective tissue through which the lymph passes.

Lymphoid tissue can either be structurally well organized as lymph nodes or may consist of loosely organized lymphoid follicles known as the mucosa-associated lymphoid tissue.

The lymphatic system differs from the circulatory system in that the lymphatic do not form a closed ring or circuit.

Instead, begin blindly in the intercellular spaces of the soft tissues of the body.

Specialized lymphatic organs are the tonsils, thymus, and spleen.

Lymphatic system plays a critical role in homeostasis.

Importance of the lymphatic system are

Maintenance of fluid balance in the internal environment

Immunity

The high degree of lymphatic capillary permeability permits large molecular weight substances which cannot be absorbed by the blood capillary to be removed from the interstitial spaces.

Proteins that accumulate in the interstitial spaces can return to the blood only via lymphatic vessels.

Absorption and transport of dietary fat, in which special lymphatic capillaries (lacteals) receive all absorbed fat (chyle) from the intestine and convey it through the thoracic duct to the venous system.

Formation of a defense mechanism for the body.

When foreign protein drains from an infected area, antibodies specific to the protein are produced by immunologically competent cells and/or lymphocytes and dispatched to the infected area.

Lacteals (lymphatics in the villi of the small intestine) serve an important function in the absorption of fats and other nutrients.

Much of this interstitial fluid is absorbed by tissue cells or reabsorbed by the blood before it flows out of the tissue.

A small amount of interstitial fluid is left behind.

If this would continue over even a brief period of time, the increased interstitial fluid would cause massive oedema.

This oedema would causes tissue destruction or death.

This problem is avoided by the presence of lymphatic vessels that act as ‘drains’ to collect the excess fluid and return it to the venous blood just before it reaches the heart.

Lymph has a composition comparable to that of plasma, but it is different in various parts of the body depending upon the tissue drained.

In particular, the lymph that leaves a lymph node is richer in lymphocytes.

Right lymphatic ducts

Receive lymph from the right upper quadrant and drains into the right subclavian vein.

Lymph from the entire body except for the right upper quadrant drains into the thoracic duct which drains into the left internal jugular vein.

Originates as a dilated structure the cisterna chyli in the lumbar region of the abdominal cavity.

From lymphatic capillaries, lymph flows through progressively larger lymphatic vessels to eventually re-enter blood at the junction of the internal jugular and subclavian veins

Although there is no muscular pumping organ connected with the lymphatic vessels to force lymph onward as the heart forces blood, still lymph moves slowly and steadily along its vessels.

Lymph flows through the thoracic duct and re-enters the general circulation at the rate of 125 mL/hour.

Occurs despite the fact that most of the flow is against gravity or ‘uphill’.

It moves through the system in the right direction because of the large number of valves that permit fluid flow only in one direction.

The movement is due to breathing movements and skeletal muscle contractions.

The mechanism of inspiration causes intraabdominal pressure to increase as intra-thoracic pressure decreases.

Contracting skeletal muscles also exert pressure on the lymphatics to push the lymph forward.

During exercise, lymph flow may increase as much as 10-15xs.

Segmental contraction of the walls of the lymphatics themselves, results in lymph being pumped from one valve segment to the next.

Other pressure generating factors includes

Arterial pulsations.

Postural changes.

Passive compression (massage) of the body soft tissues.

Nodes are oval-shaped or bean-shaped structures of lymphatic tissue through which lymph is filtered on its way to the venous system.

They are located along the paths of collecting vessels.

Some are as small as a pinhead and others as large as a bean.

Each lymph node is enclosed by a fibrous capsule.

Once lymph enters the node, it ‘percolates’ slowly through the spaces known as sinuses before draining into a single efferent draining vessel.

One-way valves in both the afferent and efferent vessels keep lymph flowing in one direction.

Fibrous septa or trabeculae extend from the covering capsule toward the centre of the node.

Cortical nodules found within the sinuses along the outer region of the node are separated from each other by these trabeculae.

Each cortical nodule is composed of packed lymphocytes that surround a less dense area called a germinal centre.

The centre or medulla of a lymph node is composed of sinuses and cords.

Both the cortical and medullary sinuses are lined with specialized reticuloendothelial cells (fixed macrophages) capable of phagocytosis.

When an infection is present, germinal centres form and the node begins to release lymphocytes.

Lymphocytes begin their final stages of maturation within the germinal centre of the nodule and then are pushed to the more densely packed outer layers as they mature to become antibody-producing plasma cells.

Defense functions; filtration and phagocytosis

The structure of the sinus channels within the lymph nodes slows the lymph flow through them.

This gives the reticulo-endothelial cells that line the channels time to remove microorganisms and other injurious particles (soot) from the lymph and phagocytose them.

Lymph node serve as a biological filter placed in the channel of several afferent lymphvessels.

Sometimes such large numbers of microorganisms enter the node that the phagocytes cannot destroy enough of them to prevent their injuring the node.

This may cause an infection of the node, adenitis.

Because cancer cells often break away from a malignant tumor and enter the lymphatics, they travel to the node, where they may set up new growths.

Haematopoiesis; The lymphatic tissues of lymph nodes serve as the sites of the final stages of maturation for some types of lymphocytes and monocytes that have migrated from the bone marrow.

Lymphocytes are located in many sites in the body, most obviously at strategic sites which are liable to infection.

The main areas of lymphocyte concentration are classified according to whether they are involved in lymphocyte generation (primary lymphoid organs, bone marrow and thymus) or the site of mature lymphocyte activation and initiation of an immune response (secondary lymphoid organs; e.g. lymph nodes, spleen).

The following are areas where lymph node can be found

Submental and submaxillary groups

Located in the floor of the mouth

Drain lymph from the nose, lips, and teeth drains through these nodes

Superficial cervical lymph nodes

In the neck along the sternocleidomastoid muscle

Drain lymph from the head and neck

Superficial cubital or supratroclear lymph nodes

Located just above the bend of the elbow

Drain lymph from the forearm passes through these nodes.

Axillary lymph nodes

20-30 large nodes clustered deep within the underarm and upper chest regions

Drain lymph from the arm and upper part of the thoracic wall, including the breast, drains through these nodes

Inguinal lymph nodes

Found in the groin

Drain lymph from the leg and external genitals drains through these nodes

Masses of lymphoid tissue located in a protective ring under mucous membranes in the mouth and back of the throat.

Help protect against bacteria that may invade tissues in the area around the openings between the oral and nasal cavities.

Palatine tonsils; located on each side of the throat.

Pharyngeal tonsils; also known as adenoids are near the posterior openings of the nasal cavity.

Lingual tonsils; located near the base of the tongue

Tonsils serve as the first line of defense from the exterior and as such are subject to chronic infection.

Thymus

Is located in the mediastinum, extending up into the neck as far as the lower edge of the thyroid and inferiorly as far as the fourth intercostal cartilage.

The pyramid-shaped lobes of the thymus are subdivided into small lobules by connective tissue septa that extend inward from a fibrous covering capsule.

Both cortex and medulla are composed of lymphocytes in an epithelial framework.

Medullary tissue can be identified by the presence of rather large laminated spherical structures called thymic corpuscles or Hassall’s corpuscles.

Thymus performs two important functions

Lymphopoies: In the embryo and newborn, eith the resulting lymphocytes migrating from the thymus to seed the spleen, lymph nodes and other lymphoid organs.

The fetal bone marrow forms immature lymphocytes which then move to the thymus.

Hormonal: It produces hormones that enable lymphocytes to develop into mature T-cells.

Spleen

Spleen is located in the left hypochondrium directly below the diaphragm, above the left kidney and descending colon, and behind the fundus of the stomach.

It is roughly ovoid.

Is surrounded by a fibrous capsule with inward extensions that roughly divide the organ into compartments.

Arteries leading into each compartment are surrounded by dense masses (nodules) of developing lymphocytes.

Because of its whitish appearance, this tissue is called white pulp.

Near the outer regions of each compartment is tissue called red pulp made up of fine reticular fibres submerged in blood that comes from nearby arteries.

After passing through the reticular meshwork, blood collects in venous sinuses and then returns to the heart through veins.

Defence

As blood passes through the sinusoids, reticulo-endothelial cell (macrophages) lining these venous spaces remove microorganisms from the blood and destroy them by phagocytosis.

Hematopoiesis

Monocytes and lymphocytes complete their development to become activated in the spleen.

Before birth, red blood cells are also formed in the spleen.

Red blood cell and platelet destruction.

Macrophages lining the spleen’s sinusoids remove worn-out red blood cells and imperfect platelets from the blood and destroy them by phagocytosis.

Also break apart the haemoglobin molecules from the destroyed red blood cells and salvage their iron and globin content by returning them to the blood stream for storage in bone marrow and liver.

Blood reservoir

Pulp of the spleen and its venous sinuses store considerable blood which can be released during haemorrhage.

Lymphatic system consists of lymph fluid, lymphatic vessels and lymphatic organs.

Main function of lymphatic system is fluid recovery, immunity and lipid absorption.

Lymphatic flow is under forces similar to those govern venous flow.

There other lymphoid tissues that functions as lymphatic system.