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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 plasma; however the concentrations of sodium, potassium, calcium and magnesium ions are mildly decreased, whilst chloride and bicarbonate ions are mildly elevated 5,6.
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, and thence to the liver. Thus the gastrointestinal and hepatic lymph takes on a milky coloration due to its high fat concentration (>0.01 g/mL triglycerides as chylomicrons), and is known as chyle.
The lymph in efferent lymphatics (postnodal) tends to be much more concentrated than the fluid in the prenodal afferent lymphatics, due to substantial absorption of water in the lymph nodes.
Other constituents of lymphatic fluid will include simple sugars, fatty acids, amino acids and cellular metabolic products 6.
History and etymology
The term lymph with its medical meaning first appears in 1725; prior to this it was a synonym of water. The word derives from the Latin term lympha meaning nymph or in a poetic sense, clear water. This was derived from the Greek term "νυμφη" (nymphe), a god-like spirit of a natural spring 7,8.