Human lymphatic System

The lymphatic system circulates lymph in the human body. The cells of human body are surrounded by the lymph. Lymph is a part of extracellular fluid in which the human cells bathe. Apart from lymph the extracellular fluid also contains Plasma and Transcellular fluids like Ocular fluid, Joint fluid and Cerebrospinal fluid. The lymph is formed of interstitial fluid. An adult has around 10 liters of lymph in his/her body. The lymph consists of salts, sugars, amino acids, hormones,coenzymes, neurotransmitters, fatty acids and the metabolic waste products. The composition of lymph varies in different tissues. The major functions of lymph are:

1. Drainage- Removal of metabolic waste,

2. Transmittive- To provide for intercellular communication

3. Nutritive- To deliver required materials to the cells, and

4. Defense- To strengthen the immune system of the body (as the lymphoid tissues contain lymphocytes)

Circulation of the lymphatic system

Though the lymphatic system is a part of circulatory system, it has no central pump and does not have an enclosure system.The movement of lymph occur through peristalsis. Other factors that contribute to lymph movement are contraction of arteries and skeletal muscles and the operation of valves.

The blood does not come in direct contact with the cells and tissues of the human body. The constituents of the human blood first exit the blood vessels and then form the interstitial fluid (lymph). The lymph then carries out cellular exchanges. The lymph vessels drains into the lymphatic duct which combine and pour their materials into the sub-clavian vein.

Organs of lymphatics System

Like other body systems, the lymphatic system consists of organs and vessels. The major lymphatic system organs are listed below.

1. Lymphatics or lymph vessels
2. Lymph nodes
3. Spleen
4. Thymus gland

 

We will look at the functioning of the various organs of the lymphatic system one by one.

The Lymphatics or The Lymph Vessels

The lymphatics vessels are closed end vessels. They are also called lymphatic capillaries. The lymphatics are found in the intercellular spaces. The walls of the lymphatics are formed of the fibrous connective tissues. The smaller lymphatics join to form larger lymphatics. The lymphatics gradually increase in size when they pass through lymph nodes, as at lymph node more lymphatics tributaries join each other. The total lymph content of the body gets connected into two major channels which are the thoracic duct and the lymphatic duct. The thoracic duct opens in the left sub-clavian vein and the lymphatic duct opens in the right sub-clavian vein.

The right lymphatic ducts- The length of the right lymphatic duct is around 1.25 cm. It drains the lymph of the right side of body areas including the right neck, chest and right forelimbs.

The thoracic duct- The length of the thoracic duct can vary from 3.8 to 45. The diameter of the duct is 4 to mm. The thoracic duct forms out of the Cisterna Chili (a dilated sac found at the lower end of the thoracic duct). It drains lymph from the left side of neck, chest and limbs in the left sub-clavian vein. The lymph of the hind limbs and the alimentary canal is received in the Cisterna Chili.

The lymphatics have valves, which aid the lymph flow and provide it proper direction. Lacteals are those lymphatics that remain in the small intestinal villi. The lacteals contain Chyle, a white fluid that is similar to lymph except that it is high in fat content. The CNS does not contain lymph vessels. The other components of human body which do not contain lymphatics are eye ball, internal ears, spleen and epidermis. The lymphatics carry tissue fluid to veins. They also carry protein and water from the interstitial fluid to blood, when they return. The lacteals help is absorption of food materials (especially fats) during digestion.

Features of Lymph

Lymph is modified tissue fluid and is contained in lymphatics. The lymph appears water like. The lymph fluid is formed by the exchange of substances between the tissue spaces and the blood capillaries. The process of exchange is called Transudation.

Composition of the lymph fluid

• The components of lymph can be divided into cellular and non cellular material.
• The cellular component consists of lymphocytes. The strength of leucocytes in lymph ranges from 500 per cu.mm. To 75000 per cu.mm.
• The non cellular of components of lymph are water and solids.
• The solids non cellular component of lymph consists of proteins namely fibrogen, globulin, albumin and traces of prothrombin (2 to 4.5%), fats (5 to 15%), carbohydrates (132.2 milligram per 100 ml of lymph and other constituents (present in small amounts) like phosphorus, creatinine, calcium, enzymes and antibodies.

Factors responsible for formation of lymph

Lymph is formed out of the fluid of tissues inside the human body. Thus, any process or thing that increases the amount of tissue fluid in the body will also contribute to increase in lymph. The lymph formation is not dependent on any secretory process or gland. It depends on physical processes and factors. The factors which contribute to the formation of lymph are described below.

• Capillary pressure- With increase in capillary pressure, the formation of lymph increases.
• Capillary wall  permeability- With increase in capillary permeability, the lymph formation rate gets increased
• Rise in temperature- With increase in temperature, the permeability of capillary walls of tissue also increases. Thus, lymph formation is increased.
• Decrease in oxygen supply to body and blood- The permeability of the vessels increase with decrease in oxygen supply (due to damage in capillary endothelium). This causes a raise in lymph formation.
• Alteration in osmotic pressure- A reduction in the colloidal osmotic pressure of the blood will result in increased lymph formation. The osmotic pressure of the blood and lymphatics varies with the amount of inorganic and organic substances present in them.
• Increase in metabolic activity of an organ- When any organ functions at a high rate, then the metabolic wastes generated increases the osmotic pressure of the tissue fluid. The temperature of the area also increases. There is a lack of oxygen and enhancement of capillary pressure. These all factors contribute to increase in formation of lymph and flow of lymph to that body area.

Circulation of Lymph in the Human Body

As we know, the lymph is not enclosed in some specific vessels like blood nor has it any pump like heart. Then how does the lymph flow across the body?

The flow of lymph in humans depends on 3 factors/things. These are:

• Presence of Valves- The valves cause the lymph to the flow in the required singular direction.

•  Pressure Gradient- The pressure difference between the tissue and the lymphatics contribute to the flow of lymph in the required direction.

•  Muscular or Skeletal Action/Contraction- The contraction of muscles causes the contraction of Lymphatics. This results in the onward flow of lymph as the valves do not allow the lymph to flow in the backward direction.

•  Movements of body parts during respiration- When we breathe in (inspiration), the diaphragm descends. This causes a fall in the intra thoracic pressure in the thoracic region. Due to fall in pressure, a pressure gradient is created which causes the movement of lymph from the lymphatics into the thorax. Next, there is a rise in the intra-abdominal pressure which results in the compression of Cisterna Chili. This increases the flow of lymph through the Thoracic Duct.

The Lymph Gland or the Lymph Node

The lymph node has a bean shape. It is small and is enclosed in a connective tissue that forms a capsular covering over it. Form the capsule of the node, small strands made of tissues pass into the nodular substance. They are called trabeculae which branch extensively and repeatedly. The trabeculae combine with the nodular reticular tissues to form sinuses. These sinuses contain the lymphoid cells. We can divide the lymph node into two regions, namely the Cortex and Medulla.

The Cortex of Lymph Node- In the cortex of lymph node, the lymph tissues are not scattered. Here, they are found in follicles or the lymphoid nodules. The diameter of the follicle can vary from .35 to 1 mm. The central area of the cortex contains Lymphocytes and is called the Secondary or Germinal nodule. The peripheral area of the cortex is called the Primary or Cortical Nodule. Blood sinuses separate the trabeculae and the lymph nodule. New lymphocytes are formed in the germinal area of the cortex. When a pathological condition occurs, a large number of macrophages get formed in this germinal area which is called the reaction center at that time. Macrophages engulf and destroy the foreign pathological substances.

The Medulla of the Lymph Node- The medulla does not has any lymphatic nodules and is less dense than the cortex. The lymph cells found in medulla are scattered. Apart from lymph cells there are also different kinds of reticulo endothelial cells found in the medulla. The trabeculae are irregularly arranged. The lymph cords of the trabeculae run in the medulla and form the communicating divisions. The lymph cords and trabeculae are separated by the Lymph Sinuses.

Hilus or Hilum- Hilus is the name given to the depression that is found in the lymph node. At this juncture, the bean shaped capsule of the lymph node is thicker. So at this juncture, the medulla comes to the surface as the cortex gets thin. Three vessels enter or exit the Hilus. 2 of them are.

1. Artery- It enters the lymph node through the Hilus and breaks into smaller capillaries called arterioles.  Mass of lymphatic cells surrounds these arteries.
2. Vein- The broken arteries gather again to form Venules. Small Venules combine to form the main vein. The main vein leaves the lymph node through the Hilus.

Therefore, the blood vessels enter and exit the lymph node through the Hilus. But it is not the case with the lymphatics. The afferent lymphatic vessels enter the lymph node through the outer shell of the capsule by piercing them. Once the lymphatics enter the capsule, they branch extensively, carry out exchanges and regroup to form the efferent lymphatic vessels. This efferent lymphatic vessel leaves the lymph nodes through Hilus. This is the third vessel found at the Hilus.

Functions performed by the Lymph Nodes

• They produce lymphocytes which aid to strengthen the immune system.
• The lymph node screens the lymph and engulfs the harmful pathogens.
• They filter poisonous content of the blood.
• They may also stop the spreading of cancerous cells.

Spleen

The spleen filters blood and is the largest lymphoid tissue of the human body. A human adult spleen weighs around 150 gm. It is located behind the floor of the abdomens. The spleen is located beneath the diaphragm and above the descending colon and left kidney. It is a haemopoietic organ i.e. formation of blood vessels occurs in spleen.

Structure of Spleen

The spleen is covered by capsule made of connective tissue. The peritoneum, or the outer layer of the capsule itself, is a serous membrane. A zigzag line or indent, called Hilus, is present at the medial area of the spleen. The nerves, blood vessels and lymphatics enter or exit the spleen through Hilus. The inner surface of the spleen (under the capsule) consists of many trabeculae. The trabeculae divide the spleen in many compartments. These tiny compartments are called lobules. Splenic pulp is the name given to the masses of parenchymal tissues present inside the capsule. The Splenic pulp is of 2 types viz. red and white. The red pulp consists of atypical lymphatic tissue and the white pulp consists of typical lymphatic tissue. The white pulp surrounds the arteries of the plasma cells, lymphocytes, macrophages and some other types of cells. The red pulp consists of the splenic sinuses and the splenic cords. The region between the junctional region of red and white pulp is called marginal zone.

Nature of circulation in the spleen

Arteries enter the spleen through the Hilus. After entering, the arteries divide themselves into trabecular branches. The branches then enter the Splenic parenchyma. The branches then reduce into reticular tissues and are infiltrated with lymphocytes.

Thus, after leaving the trabeculae the arteries enter the white pulp. Here they start branching. The branches enter the red pulp. In the red pulp the branches are sub branched and are called penicillar vessels. The long portion of penicillar vessels (made up of smooth muscles) is called pulp arteriole. The middle portion (a thick sheath) is called sheathed arteriole, and the terminal portion is called arterial capillary.

Functions of spleen

Spleen performs various functions. The major function s of spleen are listed below.

• The spleen functions as haemopoietic organ in embryo where the formation of blood vessels takes place.
• Destruction of old blood (red and white) cells.
• Spleen acts as a reservoir of blood.
• Spleen is a chief site for immune cell formation.

Thymus gland

It is a partly an endocrine gland and partly a lymphoid structure. It is located in the thorax on the back of the sternum. It has 2 flask shaped lobes. The lobes are made up of lobules. The lobules are made of small follicles of 1mm diameter. It develops form the fourth branchial clefts. The endoderm gives rise to Hassall’s corpuscles. The thymus gland secretes thymosin or thymin hormone. The thymin is the main source of lymphocytes in the blood .It also helps in providing immunologic competence to the body.

Structure of Thymus Gland

The thymus gland has an outer capsule made of connective tissue. It has 2 lobes, each having numerous lobules. Each lobule has a dense and peripheral cortex and a loose medulla. The 2 thymic lobes are joined by central strand.

The capsular region of the thymus consists of white connective tissue having plasma cells, macrophages, granular leucocytes mast cells and fat cells. The connective tissue is initially continuous with the capsule. Later, it dips into the lobules and forms a septal or trabecular system. The lymphatic vessels, nerves and blood vessels penetrate the capsule.

Thymus cortex- The cortical region the thymus lobules lack primary follicles when compared to the other lymphatic tissues. The cortex consists of lymphocytes.

Thymus medulla- The medulla of thymus consists of branched band of thymic tissue which are broad. The branching of the medulla results in the formation of lobular patterns. The medulla consists of reticular or epithelial cells. The medulla contains lesser amounts of lymphocytes than the cortex. The medulla also contains mast cells, plasma cells, melanocytes and eosinophil cells. The characteristic feature of the thymus cells are the thymic or Hassall’s corpuscles. These thymic cells are formed of reticular cells. The internal thoracic and inferior thyroid arteries provide blood to the thymus.

Lymphatic functions of Thymus Gland

• Thymus is a source of lymphocytes as lymphocyte production occurs in thymus.
• Thymus controls and influences the immune system of the body.

 Some diseases of lymphatic system

• Oedema- swelling caused by excessive secretion of lymph
• Tonsillitis- infection found in the throat tonsils
• Hodgkin’s disease-a kind of cancer found in the lymphatic system

Glandular fever- a kind of viral infection that occurs in lymphatics.