The lymphatic system is the core component of our immune system. It is associated with the immune system in such a way that the two systems are almost indifferent. For this reason, in common usage, they're often used interchangeably.
Definition: The lymphatic system can be described as a drainage system of the human body that retrieves and carries excess fluids from tissues to the bloodstream. Functionally, this system is in close association with the circulatory system. However, unlike the cardiovascular system, the lymph drainage is a one-way flow, not a circulation.
The lymphatic system consists of all cells, tissues, and organs that contain aggregates of lymphocytes, including the lymph organs (e.g. red bone marrow, thymus gland, spleen, lymph nodes and lymphoid nodules) lymph capillaries, lymph vessels and lymph ducts.
How It WorksBecause of blood pressure in the circulatory system, constituents of blood – mainly water but also comprise solutes (e.g., electrolytes and nutrients) secreted by cells – in a regular manner exit or leak from the blood vessels and enter into the interstitial spaces (i.e., spaces between individual cells in the tissues). Once tissue fluids are out of the bloodstream and in the tissue spaces, they are called as interstitial fluid or lymph. Each day, nearly 20 liters of fluids are released in the interstitial spaces, accounting approximately 11% of all body fluids.
From interstitial spaces, the lymph is forced into the lymphatic system by the contraction of skeletal muscles during body movements. The lymphatic capillaries collect the lymph from the interstitial spaces, and through lymphatic vessels (merging lymph capillaries), it is carried to the lymphatic ducts. Interconnecting lymphatic ducts eventually dump it into the subclavian veins via one of two principal vessels, the right lymphatic duct and left lymphatic duct (also known as thoracic duct). The right lymphatic duct collects lymph from the upper right side of the body and empties it into the right subclavian vein. The thoracic duct receives lymph from the rest of the body and empties it into the left subclavian vein.
Along the way, the lymph passes through a series of lymph nodes (small, bean-shaped organs), which filter debris and foreign invaders, such as bacteria and virus, from the lymph. There are about 500-600 lymph nodes in the human body, and they are located throughout the lymphatic system.
Functions of the Lymphatic SystemThe lymphatic system maintains fluid balance in the body, filters pathogens from the blood, absorbs fatty substances from the small intestine and serves as an active supporter of the blood and circulatory system. However, among many functions of this system, the most remarkable one is defending the body against microorganisms and other harmful foreign substances or antigens. When microorganisms and other foreign invaders enter into the body, the components of lymphatic system filters these harmful invaders and release antibodies.
Image source: Bruce Blaus (Own work), CC-BY-3.0, via Wikimedia Commons