The Lymphatic System

THE LYMPHATIC SYSTEM The study of lymphatic system is called lymphatology (L. lymph = spring water, G. logos = study).

The lymphatic system permeates the entire body. It comprises of a colourless fluid, the lymph a network of fine channels, the lymphatic capillaries, tubes of varied sizes, the lymphatic vessels, and the lymph nodes. (i) Lymph. Lymph is a colourless, mobile fluid connective tissue. It is, in fact the tissue fluid that slowly drains into the lymphatic capillaries from the intercellular spaces. It is now accepted that this flow is not entirely a passive drainage. The small lymphatic vessels undergo continual rhythmic contractions, the pumping action of which causes a slight negative pressure to develop. This pressure draws the tissue fluid into the lymphatic capillaries. The lymph is usually a clear, transparent fluid. It consists of two parts : a fluid matrix, the plasma, in which float amoeboid cells, the white blood corpuscles, or leucocytes. The lymph in the lymph capillaries lacks the leucocytes, but receives them as it passes through the lymph nodes. As already mentioned, the lymph differs from the blood in lacking red corpuscles, platelets and some plasma proteins, and in having less calcium and phosphorus than the blood. It has the power of clotting like the blood. (ii) Lymphatic Capillaries. The lymphatic capillaries are blind at one end. They are interwoven with the blood capillaries, but are not connected with them. They are wider than blood capillaries and their diameter is not uniform figure. They have a very thin wall, consisting of a single layer of flat endothelial cells resting on a basement membrane. However, their endothelium is extremely thin basement membrane is poorly developed.

(iii) Lymphatic Vessels. The lymphatic capillaries join to form the lymphatic vessels. The latter resemble the veins in structure, but have thinner walls and more numerous valves. The smaller lymphatic vessels unite to form larger vessels, which in turn unite to form two main lymphatic vessels or trunks called the thoracic duct and right lymphatic duct. (a) Thoracic Duct. The thoracic duct receives lymph from the entire body except the right side of the head, neck and thorax and the right arm. It arises in the abdomen from a receptaculum chyli, into which empty the lacteals, the lymphatic vessels of the intestine. The lymphatic vessels coming from the intestine give milky look as they contain absorbed fats, hence their name lacteal. It extends upward in front of the vertebral column and opens into the left subclavian vein. (b) Right Lymphatic Duct. The right lymphatic duct receives lymph from the right side of the head, neck and thorax and the right arm. It opens into the right subclavian vein. (iv) Lymphatic Nodes. The lymphatic nodes occur at intervals in the course of the lymphatic vessels. They are masses of reticular tissues wrapped in a capsule of fibrous tissue figure. They contain lymphocytes, plasma cells and fixed macrophages. The afferent lymphatic vessels enter and the efferent lymphatic vessels leave the lymphatic nodes. Valves in these vessels control one way flow of the lymph. The lymph filter through the lymphatic nodes. The macrophages remove microorganisms, cellular debris and foreign particles from the lymph. Lymphatic nodes can detect and destroy cancer cells also. However, if cancer originates in or spreads to a lymphatic node, and is not destroyed, it spreads quickly to the rest of the body by the lymph. The lymphatic nodes also add lymphocytes and antibodies to the lymph, from which these are carried to the blood.

The lymphatic nodes are specially abundant in specific regions such as the groins, armpits and neck. In case of an infection of a hand, the lymphatic nodes in the corresponding armpit gets swollen and painful. This shows that they are doing their job – entrapping infectious microbes and checking their spread to rest of the body. Lymph Movement. The lymph flows in lymphatic vessels very slowly. Forcing out of fluid fro the blood capillaries sets up some pressure in the tissue fluid. This establishes a pressure gradient in the lymphatics, causing flow of lymph in the latter. Movements of viscera and contraction of the body muscles help considerably in squeezing the lymph along. The valves present in lymphatic vessels prevent its back flow. Movement of villi assists flow of lymph in the lacteals. Gravity helps in moving the lymph down the lymphatic vessels of head and neck, Blocked of lymphatic vessels causes oedema. Lymphatic system normally returns to the veins less than 28 grams of fluid per minute.

FUNCTIONS OF LYMPH OR LYMPHATIC SYSTEM The lymph or lymphatic system serves many functions : (i)

It drains excess tissue fluid from the extracellular spaces back into the blood.

(ii)

Some of the fluid from the digestive tract is absorbed into the lymph. The lymphatic vessels store this fluid temporarily, and release it gradually so that the kidneys do not face a sudden pressure of urine excretion.

(iii)

It carries carbon dioxide and nitrogenous waste materials that diffuse in the tissue fluid to the blood.

(iv)

It takes lymphocytes and antibodies from the lymphatic nodes to the blood.

(v)

It transport fats digested and absorbed in the intestine to the blood in the form of chylomcron droplets.

(vi)

It destroys the invading microorganisms and foreign particles in the lymphatic nodes.

(vii) It maintains the quality and quantity of the blood by restoring the fluid and solute that leave it. (viii) It brings plasma protein macromolecules synthesized in the liver cells and hormones produced in the endocrine glands to the blood. These molecules cannot pass into the narrow blood capillaries but can diffuse into lymphatic capillaries. SPLEEN The spleen is the largest component of the lymphatic system. Structure. The spleen is a large (7 – 10 cm. in diameter), bean–shaped, vascular, dark-red organ located in the abdomen just blow the diaphragm at the tail of the pancreas behind the stomach. It is enlarged in severe infection. The spleen is composed of red pulp (reticular tissue rich in RBCs) having small patches of white pulp (lymphatic nodules) scattered in it figure. The red pulp is enclosed by a capsule of white fibrous tissue. The capsule sends trabeculae into the pulp, and is surrounded by visceral peritoneum. Functions. Spleen serves many functions : (i)

Destruction of Worn-out Red Corpuscles. The worn-out red blood corpuscles are phagocytized by the free and fixed macrophages present in the spleen. On this account, spleen is often described as the “graveyard” or “slaughter house” of the worn-out erythrocytes.

(ii)

Reservoir for Red Corpuscles. When the animal is at rest and needs less oxygen due to slow metabolism, some red corpuscles are withdrawn from circulation and stored in the spleen. During active life, when the animal requires more oxygen, also in case of haemorrhage (blood loss in injury), the stored red corpuscles are released into the blood steam.

(iii)

Formation of Agranulocytes. The lymphocytes and monocytes are produced in the bone marrow and spleen besides thymus and lymphatic nodes.

(iv)

Production of Antibodies. The plasma cells present in the spleen produce antibodies, the protective proteins that provide immunity.

(v)

Storage of Iron. The iron freed from the haemoglobin of the worn-out red corpuscles is stored in the macrophages till it is needed for reuse in the synthesis of fresh haemoglobin.

(vi)

Erythropoiesis. In the embryo, the spleen produces new red blood corpuscles.

(vii) Disposal of Foreign Elements. The macrophages of the spleen engulf and destroy the foreign germs and other substances entering the blood. THYMUS Thymus is also a lymphatic organ. It lies in the supper chest near the neck. It is prominent in children but begins to degenerate in early childhood. It “educates” the lymphocytes in the foetus to distinguish body cells (self) from foreign cells (nonself).

TONSILS Tonsils too are lymphatic tissues. They are located in the throat. They do not filter lymph. They are thought to protect against infection. Lymphoid tissue is also found in Peyer’s patches and vermiform appendix.