What is a Lymphatic System?

The lymphatic system is a network of complex tissues and organs that aid in the expulsion of the toxins and waste materials from the body. The role of this system is to guard the body from foreign invaders and help to maintain the body fluid level.

The blood enters the capillary beds, the hydrostatic and colloidal osmotic pressure forces some fluid out of the blood to the interstitial fluid. Most of the fluid is reabsorbed by the venous capillaries. About 15% of it remains, containing plasma proteins. The lymphatic system collects this excess of the interstitial fluid and return to the blood stream.

What are the Components of the Lymphatic System?

The various components of lymphatic system are lymphatic vessels, lymph (fluid) and lymph nodes.

Lymphatic vessels: The lymphatic vessels start out as the lymphatic capillaries that entwine themselves with the blood capillaries. They are highly permeable. The endothelial cells comprise the wall of lymphatic capillaries connected in loose fashion and forms folds that encourage the fluid to enter and prevent from leaving. Even the proteins, bacteria and pathogen can enter.

Lymph: It is a fluid that is transported through the lymphatic system, and is similar to arteries and veins carrying blood. It contains proteins and excess fluid that has been filtered in the interstitial area, and pathogens from tissues. The interstitial fluids are reabsorbed into the lymphatic vessels, and it is sent to the venous system. The interstitial fluid along with the other components enters the lymphatic capillaries. They also combine to form lymphatic trunks which drain various parts of the body. The lymph relies on pressure changes and valves to flow in the specific direction.

Note: The lymph flow is stronger during the physical activity.

Lymph nodes: These are the main lymphoid organ. They act as filters for the lymph circulating through the lymphatic system. Lymph nodes also contain macrophages that help the immune response by engulfment of the foreign pathogens. The lymph vessels converge at the round structures and they are clustered at certain regions of the body like the knee, axillary regions and cervical regions. The fluid that is passing from the lymphatic vessels enters the nodes. The lymph nodes house WBCs (white blood cells) for generating immune responses. The lymph nodes collect the lymph from the converging lymphatic vessels and there are white blood cells ready to mount an immune response against any pathogens.

Structure of the lymph node: 

The lymph node contains trabeculae, the cortex which is packed with follicles and the medulla which consists of the medullary cords and sinuses.

Other components in the lymphatic system

Lymphoid cells are immune cells like lymphocytes. Lymphoid tissues are made of loose reticular connective tissue present all over the body except thymus and macrophages.

Note: The main role of the lymphatic system is returning the fluid to the blood which gets filtered but is not reabsorbed by the capillaries. As it is the draining fluid, it might also pick up the pathogens which should be in the interstitial regions.

Lymphoid Organs

1) The spleen: It is adjacent to the stomach and acts as the site for immune surveillance and to clean the blood of foreign matter. It has the ability to salvage iron for hemoglobin production. It can also store other components of blood and release when necessary.

2) The Thymus: It is found in the thorax region below the neck. It is the site where the lymphocytes develop. In children, it is the site of T-cell maturation.

3) Patches of lymphoid tissue: These are distributed around the particular location in the body within the mucous membrane, like the tonsils, intestinal wall and bone marrow.

4) Cisterna chyli: The lymph from intestinal trunk and two lumbar lymphatic trunks flows from the dilated sac called Cisterna chyli, which is present at the lower end of the thoracic duct. Cisterna chyli acts as a drainage for most of the lymph and also functions to transport chyli which carries antibodies and T lymphocytes, thus assisting the immune system.    

5) Thoracic duct: The largest lymphatic vessel is the thoracic duct. The thoracic duct carries the lymph containing chyle. It originates from T12 (twelfth thoracic vertebra) to the root of neck and drains the systemic circulation. The right upper regions of the body are drained by the right lymphatic duct. The thoracic duct drains the left part of the body, that is, the head, neck, the torso, abdominal region, left arm and both legs.

Functions of the Lymphatic System

• The lymphatic system reabsorbs the fluid and thus restores blood pressure.
• Lymph also collects proteins and cellular debris that is large for the veins and capillaries to collect.
• Lymphatic vessels have smooth muscles and valves that help in the movement of the lymph.
• Lymph nodes acts as filtration regions, which contain lymphocytes that help fight bacteria and virus in the body.
• The lymphatic tissue, that is, the check points present in the tonsils, spleen, look out for disease fighting pathogens.
• It also absorbs fats, insoluble fat nutrients in the digestive system.
• The lymphatic system contains small vessels called lacteals that are responsible for taking the fat out.

Context and Applications  

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors in zoology   
• Masters in General physiology  
• Masters in Biochemistry

Related Concepts

• The lymphatic system
• Human anatomy and physiology

Practice Problems

What is phagocytosis?

The macrophages that patrol for intruders engulf those foreign pathogens by a process called as phagocytosis.

What are trabeculae?

These are a dense fibrous capsule and strands of connective tissue. They help divide the lymph node into compartments. Trabeculae also functions to support the reticular fibres, that in turn support the lymphocytes.

What is the function of the thoracic duct?

Thoracic duct is the terminal part of the cisterna chyli’s efferent vessel. It returns fluid to the left fluid to the left clavian and internal jugular veins.

What is the structure of the spleen?

It has two components, the white and red pulp. White pulp houses the white blood cells. The damaged red blood cells are removed via the red spleen.