The lymphatic system is a part of the immune system, comprising a network of conduits called the lymphatic vessels that carry a clear fluid called lymph unidirectionally towards the heart. After this task is complete, 90% of this fluid returns to the circulatory system as venous blood. Also, excess fluid, and waste products are removed by the lymphatic system from the interstitial spaces between the cells. The transformation is occurred by arterial blood which carries oxygen, hormones, and nutrients for the cells. Arterial blood leaves the small arteries and flows into the tissues to reach the cells. This fluid is called interstitial fluid and carries its nutritious products to the cells and it removes waste products from the cells. The lymphatic system was first described independently by Olaus Rudbeck and Thomas Bartholin. The lymph system is not a closed system. The circulatory system processes an average of 20 liters of blood per day through capillary filtration which removes plasma while leaving the blood cells. Roughly 17 liters per day of that gets reabsorbed directly into the blood vessels. The primary function of the lymph system is to provide an accessory route for these excess 3 liters per day to get returned to the blood. Lymph is essentially recycled blood plasma. The remaining 10% of the fluid that stays behind in the tissues is known as lymph yellowish fluid. Lymph originates as plasma. The arterial blood, which flows out of the heart, slows. Thanks to this slow down, some plasmas leave the arterioles and flow into the tissues. Lymphatic organs play an important role in the immune system, having a considerable overlap with the lymphoid system. Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated with the digestive system such as the tonsils. The system also includes all the structures dedicated to the circulation and production of lymphocytes, which includes the spleen, thymus, bone marrow and the lymphoid tissue associated with the digestive system. The blood does not directly come in contact with the parenchymal cells and tissues in the body, but constituents of the blood first exit the microvascular exchange blood vessels to become interstitial fluid, which comes into contact with the parenchymal cells of the body. Lymph is the fluid that is formed when interstitial fluid enters the initial lymphatic vessels of the lymphatic system. The lymph is then moved along the lymphatic vessel network by either intrinsic contractions of the lymphatic passages or by extrinsic compression of the lymphatic vessels via external tissue forces (e.g. the contractions of skeletal muscles). Eventually, the lymph vessels empty into the lymphatic ducts, which drain into one of the two subclavian veins (near the junctions of the subclavian veins with the internal jugular veins). Like veins, the lymphatic vessels, known as lymphangions, have one-way valves to avoid any backward flow.