Lymph (also known as lymphatic fluid) is the name given to the interstitial fluid once it has passed into the lymphatic vessels.
As blood passes through capillary beds a significant proportion of the plasma is filtered into the extracellular space. Most of this filtered tissue fluid (a.k.a. interstitial fluid) passes into lymphatic capillaries and is returned by the lymphatics to the systemic circulation.
An accurate composition of the lymph has only been elucidated since the early 2000s. This was primarily due to the difficulty in accessing the tiny lymphatic vessels and obtaining enough lymphatic fluid to examine, but these practical obstacles have now been largely resolved 4.
The water, glucose and electrolyte concentrations of the lymph in the initial lymphatics are broadly similar to the plasma; however the concentrations of sodium, potassium, calcium and magnesium ions are mildly decreased, whilst chloride and bicarbonate ions are mildly elevated 5.
Clotting factors are present, hence lymph will coagulate when in vitro. In general, it also has a significant protein content, which represents those plasma proteins that cross the capillary walls and are returned to the systemic circulation by the lymphatics. The quantity of lymphatic protein is less than that in the plasma and is dependent on location 1:
- liver: 6.2 g/dL
- heart: 4.4 g/dL
- GI tract: 4.1 g/dL
- lungs: 4 g/dL
- skin: 2 g/dL
- skeletal muscle: 2 g/dL
- ciliary body: 0 g/dL
In the postprandial state, aqueous-insoluble fats pass from the gut into their lymphatics, thus this lymph takes on a milky coloration due to its high fat concentration, and is known as chyle.
- 1. Kim E. Barrett, Susan M. Barman, Scott Boitano, Heddwen Brooks. Ganong's Review of Medical Physiology 25th Edition. (2015) ISBN: 9780071848978
- 2. Aspelund A, Robciuc MR, Karaman S, Makinen T, Alitalo K. Lymphatic System in Cardiovascular Medicine. (2016) Circulation research. 118 (3): 515-30. doi:10.1161/CIRCRESAHA.115.306544 - Pubmed
- 3. Hansen KC, D'Alessandro A, Clement CC, Santambrogio L. Lymph formation, composition and circulation: a proteomics perspective. (2015) International immunology. 27 (5): 219-27. doi:10.1093/intimm/dxv012 - Pubmed
- 4. Santambrogio L. The Lymphatic Fluid. (2018) International review of cell and molecular biology. 337: 111-133. doi:10.1016/bs.ircmb.2017.12.002 - Pubmed
- 5. Breslin JW, Yang Y, Scallan JP, Sweat RS, Adderley SP, Murfee WL. Lymphatic Vessel Network Structure and Physiology. (2018) Comprehensive Physiology. 9 (1): 207-299. doi:10.1002/cphy.c180015 - Pubmed