Lymph

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Diagram showing the formation of lymph from interstitial fluid (labeled here as "Tissue fluid"). Note how the tissue fluid is entering the blind ends of lymph capillaries (shown as deep green arrows)

Lymph is the interstitial fluid found between the cells of the human body. It enters the lymph vessels by filtration through pores in the walls of capillaries. 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.

Composition

Lymph has a composition comparable to that of blood plasma, but it may differ slightly depending on the tissue served (drained). 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.

Lymph is a clear liquid that is actually blood plasma without the blood cells. Its job is to bathe the cells with water and nutrients.

Formation

Formation of interstitial fluid from blood

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.^[1] 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.^[2]

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.^[3]

Unlike the cardiovascular system, the lymphatic system is not closed and has no central pump. Lymph movement occurs despite low pressure 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.^[4]

Usually, the lymph that enters the lymph vessels from the interstitial space, does not leak back because of presence of valves. But, in case of blockage of free flow, when excessive hydrostatic pressure develops within the lymph vessels, some fluid can leak back and contribute to formation of edema.

See also

• Lymphatic system
• Lymphedema
• American Society of Lymphology
• Manual lymphatic drainage

References

1. ^ Warwick, Roger; Peter L. Williams (1973) [1858]. "Angiology (Chapter 6)". Gray's anatomy. illustrated by Richard E. M. Moore (Thirty-fifth ed.). London: Longman. pp. 588–785. 
2. ^ Sloop, Charles H.; Ladislav Dory, Paul S. Roheim (March 1987). "Interstitial fluid lipoproteins". Journal of Lipid Research 28 (3): 225–237. PMID 3553402. http://www.jlr.org/cgi/reprint/28/3/225.pdf. Retrieved 2008-07-07. 
3. ^ "Definition of lymphatics". Webster's New World Medical Dictionary. MedicineNet.com. http://www.medterms.com/script/main/art.asp?articlekey=4217. Retrieved 2008-07-06. 
4. ^ Shayan, Ramin; Achen, Marc G.; Stacker, Steven A. (2006). "Lymphatic vessels in cancer metastasis: bridging the gaps". Carcinogenesis 27 (9): 1729. doi:10.1093/carcin/bgl031. PMID 16597644. http://carcin.oxfordjournals.org/cgi/content/full/27/9/1729.