Lymph is a fluid that flows through the lymphatic system, a system composed of lymphatic vessels (channels) and intervening lymph nodes, whose function, as the venous system, is the return of fluid from tissues to the central circulation. Interstitial fluid between cells in all body tissues enters the lymphatic capillaries. This lymph fluid is then transported by the larger lymphatic vessels through the lymph nodes, where the substances are removed by the tissue lymphocytes, and circulating lymphocytes are added to the fluid, before finally emptying into the right or left subclavian vein, where it is mixed with central blood.
The Lymph function restores proteins and excess interstitial fluid to the bloodstream. Lymph can collect bacteria and carry them to lymph nodes where they are destroyed. Metastatic cancer cells can also be transported through the lymph. Lymph also carries fats from the digestive system (starting from lacteals) into the blood via chylomicrons.
The tubular vessels transport the lymph back into the blood, eventually replacing the volume lost during interstitial fluid formation. These channels are lymphatic channels or simply lymphatic vessels.
Unlike the cardiovascular system, the lymphatic system is not closed and there is no central pump or lymphatic heart (which can be found in some animals). That is why lymph transport is slow and sporadic. Despite the low pressure, lymph movement occurs due to peristalsis (lymph drive due to alternating contraction and relaxation of smooth muscle tissue), valves and pressure during contraction of adjacent skeletal muscles and arterial pulsation.
The lymph that enters the lymphatic vessels from the interstitial space usually does not flow back along the vessels due to the presence of valves. If excessive hydrostatic pressure develops in the lymph vessels, some of the fluid may leak back into the interstitial space and contribute to the formation of edema.
Blood provides nutrients and important metabolites to tissue cells. It receives the waste products they produce, which requires the exchange of appropriate components between blood and tissue cells. This exchange is not direct, but it is mediated by an intermediary called interstitial fluid that occupies spaces between cells. Because the blood and surrounding cells constantly add and remove substances from the interstitial fluid, its composition is constantly changing.
Water and dissolved substances can pass between interstitial fluid and blood through diffusion through the fissures in the walls of capillaries called intracellular splits; in this way, the blood and interstitial fluid are in a dynamic balance with each other.
Interstitial fluid forms in the arterial (heart-shaped) end of the capillaries due to the higher blood pressure compared to the veins, and most of them return to the venous ends and veins; the rest (up to 10%) enter the capillaries of the lymph as lymph. Thus, the lymph, when it forms, is a watery clear liquid with the same composition as the interstitial fluid. However, when it flows through the lymph nodes, it comes in contact with blood and tends to accumulate more cells (especially lymphocytes) and proteins.