Lymph

Lymph is considered a part of the interstitial fluid, the fluid which lies in the interstices of all body tissues. Interstitial fluid becomes lymph when it enters a lymph capillary. The lymph then travels to at least one lymph node before emptying ultimately into the right or the left subclavian vein, where it mixes back with blood.

Lymph returns protein and excess interstitial fluid to the circulation. Lymph picks up bacteria and brings them to lymph nodes to be destroyed. Metastatic cancer cells can also be transported via lymph. Lymph also transports fats from the digestive system.

The word is derived from the name of the Roman divinity, Lympha.

Composition
Lymph has a composition comparable to that of blood plasma, but it may differ slightly. Lymph contains white blood cells. In particular, the lymph that leaves a lymph node is richer in lymphocytes. Likewise, the lymph formed in the digestive system called chyle is rich in triglycerides (fat), and looks milky white.

Formation


Blood supplies nutrients and important metabolites to the cells of a tissue and collects back the waste products they produce, which requires exchange of respective constituents between the blood and tissue cells. However, this exchange is not direct but instead is effected through an intermediary called interstitial fluid or tissue fluid that the blood forms. Interstitial fluid (ISF) is the fluid that occupies the spaces between the cells and constitutes their immediate environment. As the blood and the surrounding cells continually add and remove substances from the ISF, its composition continually changes. Water and solutes can freely pass between the ISF and blood via diffusion across capillary walls and thus both are in dynamic equilibrium with each other.

ISF forms at the arterial (coming from the heart) end of capillaries because of the higher pressure of blood compared to veins, and most of it returns to its venous ends and venules; the rest (1%) enters the lymph capillaries as lymph. Thus, lymph when formed is a watery clear liquid with the same composition as the ISF. However, as it flows through the lymph nodes it comes in contact with blood, and tends to accumulate more cells (particularly, lymphocytes) and proteins.

The two primary lymph systems are the thymus gland and the bone marrow, where the immune cells form or mature. The secondary lymph system is made up of encapsulated and unencapsulated diffuse lymphoid tissue. The encapsulated tissue includes the spleen and the lymph nodes. The unencapsulated tissue includes the gut-associated lymphoid tissues and the tonsils.

Lymphatic circulation
Tubular vessels transport lymph back to the blood, ultimately replacing the volume lost during the formation of the interstitial fluid. These channels are the lymphatic channels, or simply lymphatics.

Unlike the cardiovascular system, the lymphatic system is not closed and has no central pump. Lymph transport, therefore, is slow and sporadic. Despite low pressure, lymph movement occurs due to peristalsis (propulsion of the lymph due to alternate contraction and relaxation of smooth muscle), valves, and compression during contraction of adjacent skeletal muscle and arterial pulsation.

Lymph that enters the lymph vessels from the interstitial space usually does not flow backwards along the vessels because of the presence of valves. If excessive hydrostatic pressure develops within the lymph vessels, though, some fluid can leak back into the interstitial space and contribute to formation of edema.

As a growth medium
In 1907 the zoologist Ross Granville Harrison demonstrated the growth of frog nerve cell processes in a medium of clotted lymph.

In 1913, E. Steinhardt, C. Israeli, and R. A. Lambert grew vaccinia virus in fragments of tissue culture from guinea pig corneal grown in lymph.