[Biology Class Notes] on Lymphatic System Pdf for EXAM

Have you ever wondered how the food we eat is circulated around the whole body? Or when we take medicine or get an injection, how are they circulated to the part that needs them? The answer to the above questions is the lymphatic circulatory system. It is a subsystem of the circulatory system and is composed of tissues and organs. It is also known as the lymphoid system and is an essential component of the immune system. Below we will learn more about the Lymphatic System.  

What is the Lymphatic System?

This system is present in the body of vertebrates and is composed of vessels, tissues, and organs. The human Lymphatic System helps in maintaining fluid balance. It does so by collecting the excess fluid from tissues and then depositing it into the bloodstream. The lymph circulatory system also generates lymphocytes that help in fighting against diseases. The fluid that runs through the Lymphatic System is known as lymphatic fluid.

Lymphatic System Structure

The Lymphatic System in the human body is made up of lymph, lymphatic vessels, lymphatic nodes, and lymphoid tissue. Lymph is a colorless, watery fluid that mainly consists of white blood cells and is carried by the Lymphatic System. The lymph vessels are the site of fluid drainage. They pump the lymph fluid with the help of skeletal and smooth muscles. To prevent the backflow the larger lymph vessels are present with valves. A lymph node is nothing but a mere collection of lymphoid tissue. They are located at regular intervals in the Lymphatic System. Lymphoid tissue is composed of lymphocytes along with other specialized cells and tissues that help in maintaining the immune system’s functions. The Lymphatic System diagram is shown below.

Figure: The Lymphatic System Inhumans

Lymphatic Circulation

After reading the above information you must be thinking about where does lymph drain? Well, the answer to this question will be covered under this topic. The Lymphatic System can be thought of as a drainage system that is needed by the body because blood circulates and its plasma is leaked into the thin walls of the capillaries. Extracellular fluid is the portion of the blood plasma that escapes and it contains oxygen, amino acid, and other useful nutrients that are needed by the body and tissue cells. Almost all of the fluid gets back into the bloodstream but a small percentage of it along with the particulate matter is left behind. Here then comes the role of the Lymphatic System which helps in removing these fluids and materials from tissues which thus preventing the fluid imbalance in the body. This imbalance can lead to the death of the organism. This lymph gets drained into larger vessels that are known as lymphatic vessels and these vessels converge to form the lymphatic trunk. These lymphatic trunks are connected to the veins and thus the excess materials or the infectious microorganisms are removed via this pathway. These lymphatic vessels are punctuated at intervals by small masses of lymph tissue which are known as lymph nodes.

Lymphoid Organs

The body lymph system is divided into primary lymphoid organs and secondary lymphoid organs. The primary lymphoid organs are the sites of B and T cells maturation sites and the secondary lymphoid organs are where these cells are further differentiated and perform their functions. The thymus, bone marrow, fetal liver are the primary lymphoid organs. Thymus and bone marrow are the major key players in immune function. The cells that mature in the bone marrow are termed B cells and the cells that migrate from bone marrow to thymus are called T cells. When these B and T cells mature they are migrated to the secondary lymphoid organs via the bloodstream where they are activated by coming in contact with the foreign materials which are termed as antigens. Lymph nodes, spleen, tonsils are termed secondary lymphoid organs. This is where these matured cells perform their function.

The human body functions perfectly with a lot of internal systems and mechanisms in place. Some of the common systems that we all are aware of are the digestive, circulatory, nervous, respiratory, excretory systems, etc. Apart from this, there is another important system called the Lymphatic System or lymph, which plays a major role in the functioning of the body.

The Lymphatic System

The Lymphatic System in vertebrates is also called a subsystem of the circulatory system. It has a complex network of lymphatic vessels, tissues, and organs. Transportation of the lymph throughout the body is the primary role of this system. It also helps to maintain the fluid balance in the body, in which it collects the excess amount of fluid and other particulate matter from the tissues and stores it in the bloodstream. With the help of lymphocytes (the disease-fighting cells), It defends the body against infections.

Lymphatic Circulation

The Lymphatic System is also referred to as a drainage system that is essential to circulate the blood throughout the body, blood plasma through the thin walls of the capillaries gets leaked into the tissues. A portion of the fluid along with the particulate matter gets left behind, as most of the fluids immediately seep back into the bloodstream. The fluid and these materials from tissues are removed by the Lymphatic System by returning them to the bloodstream through the lymphatic vessels and thereby preventing a fluid imbalance that would probably result in the death of organisms.

The fluid and proteins within the tissues get back to the bloodstream through tiny lymphatic capillaries by which most of the tissue in the body gets infused. There are only a few regions that are free from lymphatic capillaries such as the epidermis of the skin, the mucous membranes, the bone marrow, and the central nervous system, whereas the regions that are densely packed with these vessels are lungs, gut, genitourinary system, and dermis of the skin. The extracellular fluid within the Lymphatic System known as lymph drains into larger vessels called lymphatics. 

These vessels form one of two large vessels when converged. These vessels are also called lymphatic trunks, that connect veins at the base of the neck. The right lymphatic duct drains the upper right portion of the body through the right subclavian vein and returns lymph to the bloodstream. The thoracic duct drains the other part of the body into the left subclavian vein. By muscle contractions, the lymph along with the system of vessels gets transported, and valves prevent lymph from flowing backward. By small masses of lymph tissue, the lymphatic vessels get punctuated at intervals, called lymph nodes, which on lymph filtering removes foreign materials like infectious microorganisms.

Lymphoid Organs

The Lymphatic System is broadly classified into two organs, namely primary lymphoid organs and secondary lymphoid organs. The primary lymphoid organs are those which have the sites of B and T cell maturation, whereas the secondary lymphoid organs are those in which further differentiation of lymphocytes occurs. The tand, thymus, bone marrow, fetal liver, are the primary lymphoid organs in humans, Whereas in birds there is a structure called the bursa of Fabricius. In humans, the thymus and bone marrow play a major role in the function of the immune system. All lymphocytes are produced from the stem cells in the bone marrow. 

Stem cells are said to become B lymphocytes and are said to remain in the bone marrow due to their maturity, whereas the prospective T cells travel to the thymus to undergo further growth. Mature B and T lymphocytes leave the primary lymphoid organs and transport the rest to the secondary lymphoid organs through the bloodstream, which when contacted with foreign materials such as particulate matter and infectious agents are activated. They are called antigens in this context.